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REESE LIBRARY 



UNIVERSITY OF CALIFORNIA. 

, ,8gf. 



Accessions No. fif.J.btf. ... Class Mi. 

i-ir-u 




PAI NTERS' 

COLOURS, OILS, AND VARNISHES 

A PRACTICAL MANUAL. 



CHARLES GRIFFIN & COMPANY'S 
STANDARD TECHNOLOGICAL HANDBOOKS. 



OILS, FATS, WAXES, and Allied Materials, and the Manufacture 
therefrom of CANDLES, SOAPS, and OTHER PRODUCTS. 
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A. JAMIESON, M.Inst.C.E., F.R.S.E., &c., Professor of Engineer- 
ing, Glasgow and West of Scotland Technical College. With over 
200 Illustrations, Six Folding Plates, and Examination Papers. 
SEVENTH EDITION, 8s. 6d. 

" The BEST BOOK yet published for the use of students.'' Engineer. 



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By R. D. MUNRO, Engineer of the Scottish Boiler Insurance Com- 
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ought to be carefully studied, and ALWAYS AT HAND." Colliery Guardian. 



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Apparatus, and Materials. Comprising Working Details of all the 
more important Methods. By A. BROTHERS, E.R.A.S. With 24 
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LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. 



PAINTERS' 



COLOURS, OILS, AND VARNISHES 



A PEACTICAL MANUAL. 



BY 



GEORGE H. HURST, F.C.S., 

MEMBER OP THE SOCIETY OF CHEMICAL INDUSTRY; 

LECTURER ON THE TECHNOLOGY OF PAINTERS* COLOURS, OILS, AND VARMSHK.- 
AT THE MUNICIPAL TECHNICAL SCHOOL, MANCHESTER. 



TIHUt b mumerous ^lustrations. 



LH 

or THE 

tTNIVERSITT, 



LONDON: 

CHARLES GRIFFIN & C,OMPANY, LIMITED, 

EXETER STREET, STRAND. 

1892. 

[All Rights Reserved]. 



PREFACE. 



IN offering the following pages to Practical "Workers and others 
interested in the wide subject of "Painters' Colours, Oils, and 
Varnishes," my aim throughout has been to combine theory 
and practice, and to show the scientific principles that underlie 
the methods in constant use. Naturally and one may say 
unavoidably there has grown up in the course of years, in 
connection with Colour-making, as with every other industry, 
a good deal of what is known as " Rule of thumb " procedure. 
The amount of this that prevails, however, has been greatly 
overrated, and we are not far distant from the day when 
" Rule of thumb " will be generally supplemented among us by 
an intelligent appreciation of the scientific principles involved. 
To give the rationale of every technical process is, nevertheless, 
by no means an easy task, and all that I can hope to have 
effected is the placing before the reader such a description of 
the various processes and their underlying principles, as shall 
be really helpful in practical work. 

The information given as to the properties and preparation 
of Pigments, is either based on. my own experience, or drawn 
from the most trustworthy sources. For a revision of the 
chapter on Varnishes, and many excellent suggestions, I am 

indebted to a personal friend, practically engaged in their 

b 



VI PREFACE. 

manufacture. My best thanks are due to him, and also to 
Messrs. Brinjes & Goodwin, Follows & Bate, Ritchie & Co., 
Rose, Downs & Thompson, and Rushton, Irving & Co., who 
have kindly furnished for the work illustrations of the newest 
types of Paint and Oil Machinery. 



GEORGE H. HURST. 



CHEMICAL LABORATORY, 

22 BLACKFRIARS STREET, 

MANCHESTER, October, 1892. 




GENEBAL CONTENTS. 



CHAPTER I. 
INTRODUCTORY. 

PAGES 

Colour, the Spectrum Colours, White from Coloured Light, Light 
from Coloured Bodies, Cause of Colour in Coloured Bodies, 
Colour Theories, Pigments, Paint, Varnishes, . . 1-8 



CHAPTER II. 
WHITE PIGMENTS. 

White Lead; Manufacture of White Lead; Dutch Method of White- 
Lead making; Chemistry of White- Lead making; Chamber 
Methods of White-Lead making ; Creed or German Process, 
Hatfield Process, Thompson's Process, Gardner's Electric 
Process ; Precipitation Processes of White-Lead making ; the 
Kremnitz, Thenard, Cory, Milner, Martin, Fourmentin, 
Spence, Maclvor, Dundonald, Pattinson, Dale & Milner, 
Watt & Tebbutt, Delafield, Rowan, Lowe, and Condy Pro- 
cesses ; Miscellaneous Processes of Making White Lead ; the 
Torassa, Mullins, Martin, Lewis, Button & Dyar, Brown 
& Young, Maxwell Lyte, Woolrich, Ozouf, and Cookson 
Processes; Composition and Properties of White Lead; 
Analyses of White Leads; Assay and Analysis of White 
Lead ; Analysis of Dry White Lead ; Analysis of Paste White 
Lead. Sulphate of Lead Pigments ; Sublimed White Lead, 
Freeman's Non-poisonous White Lead, Hannay's Caledonian 
White Lead, Maxwell Lyte White Lead. Sulphite of Lead 
White ; Zinc Whites, Zinc White ; Sulphide of Zinc Whites, 



Vlii CONTENTS. 

PAGES 

Orr's White, Charlton White, Griffith's Zinc White, 
Knight's Zinc White, Lithophone; Barytes, Blanc Fixe, 
Gypsum, Strontian White; Whiting; Magnesite; China 
Clay; Manufacture of China Clay, Composition and Pro- 
perties of China Clay, Assay and Analysis of China Clay; 
Wilkinson's White Lead; Pattinson's White. Lead, . 9-87 



CHAPTER III. 
BED PIGMENTS. 

Vermilion; Manufacture of Vermilion, Properties of Vermilion; 
Red Lead, Manufacture of Red Lead, Properties and Com- 
position of Red Lead, Assay and Analysis of Red Lead; 
Orange Lead ; Red Oxides, Oxides of Iron, Manufacture of 
Oxide Reds, Properties of Red Oxides, Analysis and Assay 
of Red Oxides, Analyses of Red Oxides ; Antimony Vermilion, 
Properties of Antimony Vermilion, Composition of Anti- 
mony Vermilion, Analysis and Assay of Antimony Vermilion; 
Brilliant Scarlet; Chromate of Mercury; Chromate of Silver ; 
Chromate of Copper ; Magnesia Pink, . . .88-114 



CHAPTER IV. 
YELLOW AND ORANGE PIGMENTS. 

The Chromes, Manufacture of Lead Chromes, Preparation of 
Chrome Yellows, Preparation of Chrome Oranges and 
Scarlets, Preparation of Chrome Red, Properties of the 
Lead Chromes, Assay and Analysis of the Lead Chromes ; 
Zinc Chrome, Preparation of Zinc Chrome, Properties of 
Zinc Chrome, Assay and Analysis of Zinc Chrome, Lemon 
Chrome; Barium Chrome; Ochres and Siennas, Properties 
of Ochres and Siennas, Assay and Analysis of Ochres and 
Siennas, Composition of Ochres and Siennas, Burnt Sienna; 
Mars Colours, Turner's Yellow, Naples Yellow, King's 
Yellow, Realgar, Indian Yellow, Cadmium Yellow, Aure- 
olin, ....... .115-150 



CONTENTS. ix 

CHAPTER V. 
GREEN PIGMENTS. 

PAGES 

Brunswick Green, Chrome Green, Copper Greens, Verdigris, 
Scheele's Green, Emerald Green, Mineral Green, Green 
Verditer, Bremen Green, Terre Verte, Cobalt Green, 
Brighton Green, Douglas Green, Chinese Green, Sap Green, 
Manganese Green, Titanium Green, Zinc Green, . .151-179 

CHAPTER VI. 

BLUE PIGMENTS. 

Ultramarine, Natural Ultramarine, Artificial Ultramarine, Manu- 
facture of Ultramarine, Properties of Ultramarine, Com- 
position of Ultramarine, Constitution of Ultramarine; Assay 
and Analysis ; Ultramarine Derivatives, Violet Ultramarine, 
Red Ultramarine; Prussian Blues, Chinese Blue, Soluble 
Blue, Antwerp Blue, Brunswick Blue, Properties of Prussian 
Blues, Assay and Analysis of Prussian Blues; Cobalt Blues; 
Smalts, Manufacture of Smalts, Composition and Properties 
of Smalts, Assay and Analysis of Smalts; Cobalt Blue, 
Properties and Composition of Cobalt Blue, Assay and 
Analysis of Cobalt Blue; Copper Blues, Mountain Blue, 
Bremen Blue, Blue Verditer, Lime Blue, Properties of 
Copper Blues ; Coeruleum, Manganese Blue, . . 180-223 

CHAPTER VII. 
BROWN PIGMENTS. 

Umber, Composition and Properties of Umbers, Assay and Analysis 
of Umbers, Vandyke Brown, Sepia, Cappagh Brown, Man- 
ganese Brown, Bistre, ..... 224-231 

CHAPTER VIII. 
BLACK PIGMENTS. 

Lamp and Vegetable Blacks, Properties and Composition of Lamp- 
Blacks, Assay and Analysis of Lamp-Blacks ; Bone-Black, 



CONTENTS. 

PAGES 

Properties and Composition of Eone- Black, Assay and 
Analysis of Bone-Black, Ivory-Black ; Animal Black, Frank- 
fort Black, Miscellaneous Blacks, . . . . 232-248 



CHAPTER IX. 
LAKES. 

Red Lakes, Carmine, Carmine Lake, Florentine Lake, Brazil- Wood 
Lakes ; Rose Pink, Red Lake, Yellow Lakes, Orange Lake, 
Madder Lakes, Madder Red Lakes, Green Lakes, Violet 
Lake; Analysis of Lake Colours, Reactions of Natural Dye- 
stuffs, Aniline Lakes, Coal-Tar Colouring Matters, Precipi- 
tating Agents for Aniline Lakes, Manufacture of Aniline 
Lakes, Vermilionettes, and Royal Reds, Scarlet Lakes, 
Orange Lakes, Yellow Lakes, Blue Lakes, Brown Lakes, 
Violet Lakes, Black Lake, Green Lake, Alizarin Lakes, . 249-281 



CHAPTER X. 
ASSAY AND ANALYSIS OF PIGMENTS. 

Colour and Hue, Brilliancy or Luminosity, Colouring Power, Cover- 
Power or Body, Durability or Permanence, Mixability, 
Fineness, . . . . . . . 282-293 

CHAPTER XL 
COLOUR AND PAINT MACHINERY. 

Levigation, Drying of Pigments, Preparing Pigments or Colours 

by Precipitation, Filtering, Grinding, Mixing, . . 294-330 



CHAPTER XII. 
PAINT VEHICLES. 

Paint Oils, Drying Oils, Linseed Oil, Boiled Oil, Rosin Oil, Turpen- 
tine, Kosin Spirit, Shale Spirit, Benzoline, Coal-Tar Naphtha, 
Methylated Spirit, ...... 331-383 



CONTENTS. xi 

CHAPTER XIII. 
DRIERS. 

PAGES 

Red Lead, Litharge, Manganese, Mixed Driers, . . . 384-388 

CHAPTER XIV. 
VARNISHES. 

Varnish Materials, Drying Oils, Resins, Oil-varnish Resins, Ethereal - 
varnish Resins, Spirit-varnish Resins; Gums; Colouring 
Matters ; Artificial Colouring Matters, Coal-tar Colours ; 
Varnish Making ; Natural Varnishes, Oil Varnishes, Spirit 
Varnishes, Water Varnishes, .... 389-450 

INDEX, ........ 451 




PAINTERS' COLOURS, OILS, 
AND VARNISHES. 



CHAPTER I. 

INTRODUCTORY. 

COLOUR, COLOURS, PAINTS, AND VARNISHES. 

Colour is a term used by persons in several senses; hence 
confusion sometimes arises, although, as a rule, the context 
leaves no doubt as to the particular sense intended. When a 
beam of white light is made to pass through the angle of a tri- 
angular prism in a certain manner, and the light which has 
passed through is received upon a screen, we find that it has under- 
gone a wonderful change ; instead of being one uniform colour, 
as it was originally, it is spread out into a band of many colours, 
of which seven can readily be distinguished viz., red, orange, 
yellow, green, blue, indigo, and violet. We see these colours by 
the effect or sensation produced by their action on the retina of 
the eye ; in a sense, therefore, these colours have an abstract 
existence only, we can see them by the eye, but we cannot 
handle them as we can a piece of cotton. When we speak of a 
red colour or a green colour, we use the term " colour " in an ab- 
stract sense to indicate the sensation which these colours create 
in our eyes. On the other hand, we often speak of coloured 
bodies (that is bodies which give the sensation of being coloured 
when 'we look at them) as " colours," especially when (as with 
vermilion, chrome yellow, emerald green, Prussian blue, and 
magenta) they can be used to impart colour to other bodies. 
In this way "colour" is used in a concrete sense to indicate 



INTRODUCTORY. 



tangible bodies which have the power of causing other bodies to 
which they may be applied to create, so to speak, the sensation 
of colour. Although the subject is one of some importance to- 
users of colours, it is not intended to enter here into a long dis- 
cussion of colour from an abstract point of view, inasmuch as 
space does not admit of doing so in any adequate manner, the 
reader must, therefore, be referred to other works specially 
devoted to the consideration of the subject. 

The Spectrum-Colours. When, then, a beam of white light 
is passed in a particular manner through the edge of a triangular 
prism, it undergoes two changes (1) the direction of its course 
is altered, i.e., it becomes refracted ; and (2) the beam of white 

light is separated into a, 
divergent band of several 
differently- coloured 
light-rays. Fig. 1 repre- 
sents the path of a beam 
of light through a tri- 
angular prism ; a is a ray 
travelling in the direc- 
tion of the arrow which 
strikes the prism at c. If 
the prism had not been 
there, it would have 
passed on and would 
have fallen upon the 
screen, s, s, at b \ but the 
prism, bending it out of 
this course, refracts it as 
shown at c; it then passes- 
through the prism in the 
new direction until it 




Fig. 1. 



emerges at d, where it is again refracted so as to take the new 
direction, d, f. As the amount of refraction differs for each ray 
according to its colour, the result is that the original white beam 
of light is differentiated into a long band of numerous distinct 
colours, known as the spectrum, which extends from e to / in 
the screen. The rainbow is a spectrum of this kind formed by 
the refraction of the sun's light during its passage through the 
drops of water in a shower of rain. In the latter case, however, 
the spectrum is seen in front of the drops, not behind them, as 
it is formed by the rays, which, falling on the drops, pass to the 
back, and are then reflected so as to emerge again on the side 
nearest the sun. 



LIGHT FROM COLOURED BODIES. 3 

The colours of the spectrum are pure colours i.e., they cannot 
be further split up; if, say, the red part of the spectrum be passed 
through a second prism, no new colours are produced ; the light 
which passes through the second prism is still red, although it is 
distributed over a wider surface. It, therefore, follows that there 
are really a very large number of simple colours in the spectrum, 
although, owing to the limitations of language, it is impossible to 
separate and name every one of these in a popular manner; 
although scientists can do so in another manner which it is not 
necessary to describe here. It is, however, customary to follow 
the lead of Sir Isaac Newton, who discovered this property of 
white light, and to distinguish seven colours viz., red, orange, 
yellow, green, blue, indigo, and violet; but it should be distinctly 
understood that in the spectrum there is no well-marked line of 
division between these seven so-called primary colours ; the red 
passes insensibly into the orange, the orange into the yellow, and 
so on through the other colours in the order given above. 

White from Coloured Light. By passing the spectrum 
colours through a lens, or through another prism, in a particular 
manner, the seven colours can be recombined so as to form white 
light. It is not even necessary to use all the spectrum colours, 
as two or three will suffice if properly selected. Thus blue and 
yellow will when united form white light ; as also red, green, and 
blue, and many other combinations, particulars of which will be 
found in special books on Colour, such as those of Professor 
Church and Mr. W. Benson. The consideration of this property 
of a few of the spectrum colours combining together to form 
white light led Young, and, later, Helmholtz, to consider that 
there are only three primary colours, red, green, and blue, from 
which all the other colours can be obtained ; thus, by combining 
red and green, yellow is produced; or by combining red and blue 
violet is the result. 

Light from Coloured Bodies. When the light which is 
reflected from the surface of a coloured body like vermilion is 
passed through a prism, it is found to yield a spectrum ; not, 
however, a complete one, such as is got from a ray of white light, 
but one more or less incomplete ; thus, vermilion gives a spec- 
trum containing some red, orange, and a little blue light ; chrome 
yellow again gives a spectrum showing a few red, some yellow, 
and some green rays; in each case the eye distinguishes the 
effect due to the combined action of all these rays on the retina. 
No artificial colouring-matter is known which reflects rays of one 
colour only ; in every case the rays of the dominant colour are 
mingled with those of other colours. The light from some bodies 



4 INTRODUCTORY. 

is of a very complex character, while that from others is com- 
paratively simple. It is this complexity in the composition of 
the light reflected that makes it so difficult to demonstrate the 
true laws and facts of colour with pigments or any artificial 
colouring-matters. 

Cause of Colour in Coloured Bodies. The actual reasons 
why bodies such as vermilion, magenta, or emerald green are 
coloured, it is almost impossible to investigate in the present 
state of knowledge, since the cause, whatever it may be, must 
be due to the molecular construction of the different compounds 
about which very little is known ; still, we know something of 
some of the reasons why coloured bodies appear coloured. When 
light falls upon a substance, the light may be affected in one or 
two ways ; it may be reflected, that is, it may be thrown back 
from the body; or it may be transmitted, that is, it may pass 
through, or, in some cases, be absorbed by the body on which it 
has fallen. As a rule, there is never either complete reflection 
or complete transmission of light, the most perfectly reflecting 
body allowing some rays to pass into it. It is by reflected light 
that we see bodies ; when the reflection is complete, or as nearly 
so as is the case with mercury or a very highly polished plate of 
silver, the body is nearly invisible ; it is only rendered visible 
because it does not reflect all the light which falls upon it in a 
regular manner ; some is irregularly reflected and it is this light 
which enables us to see the body. Two kinds of reflection can, 
therefore, be distinguished regular and irregular. Regular reflec- 
tion is that where the light is thrown back in a straight line from 
the reflecting surface ; if this is perfect, only the light that is 
reflected is seen, the reflector itself is invisible. Irregular reflec- 
tion is that where the light is thrown back from the reflector in 
every direction; it is this light which makes the body visible, and 
it is due to the fact that no matter how apparently even the sur- 
face may appear to be, yet it is not even ; it is sufficiently rough 
to cause the light which falls upon it to be irregularly reflected. 
Then bodies never reflect or absorb the whole of the light which 
falls upon them, some of it is absorbed; the most perfectly polished 
plate of silver (which is the most highly reflecting body known) 
does not reflect the whole of the light which falls upon it, while 
a piece of black cloth reflects only a little of the light that falls 
upon it. Upon the character of the reflected light thrown off 
from a body depends its colour, which is independent of the pro- 
portion of the light that falls upon and is reflected by the body. 
If all the rays of light falling upon it are reflected, then the body 
appears white ; if all the light rays are absorbed, then the body 



COLOUR THEORIES. 5 

appears to be black. If, now, some of the spectral rays are 
absorbed and the rest reflected, then the body appears to be 
coloured, the colour depending upon the composition of the rays 
which are reflected ; thus the rays from a red body, such as ver- 
milion, are red, as are also those from Derby red and oxide of 
iron ; similarly, the rays from a yellow body, such as chrome 
yellow or yellow ochre, are yellow, but it does not follow that the 
rays from all red bodies or from all yellow bodies are identical in 
composition. If the rays from, say, vermilion, oxide of iron, 
and crimson lake are passed through a prism, and the spectra of 
the coloured light which is reflected from each examined, they 
will be found to be different; that from the crimson lake will con- 
tain more blue rays than that from the vermilion, while that 
from the oxide of iron will contain more of the dark red and 
indigo rays than either of the others ; and it is the same with 
the other classes of colours. There is no coloured body known, 
which reflects what might be called a pure light, while the 
spectrum-colours are pure, as has been already stated. It is this 
compound nature of the light which is reflected from coloured 
bodies that makes it extremely difficult to demonstrate the true 
laws of light and colour by the use of pigments. 

In the same manner as the coloured light which is reflected 
from bodies is compound, so that which is transmitted is com- 
pound and, usually, the complement of that which is reflected, 
but this does not always happen. When it is the comple- 
ment of that which is reflected, then the bodies which give rise 
to this phenomena are known as dichroic ; in other cases both 
the reflected and transmitted rays are of the same general colour, 
although there is usually some difference in the actual tint of 
the two colours. 

It is assumed that the coloured bodies have a selective action 
on the light which falls upon them, reflecting or transmitting, 
as the case may be, those coloured rays to which they owe their 
colour, while they absorb all the other rays. White bodies 
reflect all the rays which fall upon them, black bodies absorb all 
and are, in consequence, often nearly invisible. As to the 
character of the rays reflected from red, orange, yellow, green, or 
other coloured bodies, these will have been inferred from what 
has been said above. 

Colour Theories. Two theories of colour are in use to 
explain the coloured effects of light. The old theory, which is 
mostly due to Brewster, considers that there are three primary 
colours viz., red, yellow, and blue ; by the proper admixture 
of which in various proportions all the other colours can be 



6 INTRODUCTORY. 

obtained. The more modern theory, first broached by Young 
and more fully developed by Helmholtz, considers that there 
are three primary colours, red, green, and blue, although some 
authorities add a fourth. However, it must be confessed 
that while the modern theory accurately explains all the phe- 
nomena of colour producible by the use of the spectrum colours, 
yet the older theory of Brewster more easily explains the 
phenomena of colour as produced by the admixture of the 
various colouring-matters, pigments, and dyestuffs in common 
use; this arises not from any fault in the newer theory, but 
from the compound nature of the light which is reflected or 
transmitted from the colouring-matters in question. Of the 
newer theory it is not intended to deal, although it is advisable 
for colourists to make themselves acquainted with it ; as to the 
old theory, it will be sufficient to say that when any two of the 
primary colours are mixed together a so-called secondary colour 
is produced ; thus red and yellow produce orange, red and blue 
produce violet, while yellow and blue make green. When the 
secondaries are mixed together they produce what are called 
tertiary colours, of which there are six, known as buff, citrine, 
sage, slate, plum, and russet. The nomenclature of these tertiary 
colours is very indefinite, and different authorities give them 
different names. 

The common theory of red, blue, and yellow is not wholly 
satisfactory, as it does not account for all the shades which may 
be produced by the admixture of pigments ; thus a mixture of 
ultramarine, a blue, with yellow ochre, a yellow, does not 
produce a green, as the theory would expect, but a kind of 
: greenish-grey ; this effect can, however, be explained by the 
blue-red-green theory when we know the kind of rays reflected 
by the two pigments in question. Reference must be made to 
text-books on colour for a further development of the subject. 

Colours. It has been explained above that the term "colours" 
is used in two senses first, to express the sensation which light 
of various kinds evolved from bodies excites on the retina of the 
eye, and which sensation is purely functional ; second, to denote 
those bodies which, having the property of selective absorption 
of coloured rays from the light which falls upon them, appear to 
be coloured and which have the property of imparting this 
colour to other bodies ; such bodies are known as colouring 
matters and mny be divided into two groups, dyestuff's and 
pigments ; the former are mostly soluble in water and are used 
solely to dye cotton, wool, or other textile fibres, while the 
latter are insoluble, and are used in the preparation of paints. 



PAINT. 7 

Besides these two classes of coloured bodies there is another 
group which are distinguished by the fact that while possessing 
colour yet they cannot impart this colour to other bodies ; such 
are bluestone (sulphate of copper), nitrate of cobalt, chrome 
alum, &c. 

Pigments. These are a fairly numerous class of colouring 
matters which are used to give colour to paint. They are 
mostly derived from the mineral kingdom, although a few are 
obtained from organic sources. As a class they are distinguished 
by being insoluble in water, turpentine, and most other solvents 
with the exception of the strong acids ; they are opaque or 
nearly so ; and they should be perfectly inert bodies exercising 
no action of any kind on any other substance with which they 
may be mixed. As typical examples of pigments may be taken 
barytes, oxide of iron, yellow ochre, chrome green, and umber. 

In dealing with pigments in detail they will, as a rule, be 
considered under the divisions of white pigments, red pigments, 
yellow pigments, and so on ; but here and there, deviations from 
this rule will be made, as in the case of Derby red (which will 
be dealt with under the head of yellow pigments) and in the 
case of lakes, where it is thought that the composition and 
properties of the particular pigments can be more conveniently 
pointed out, if dealt with in one group. 

Paint. Paint is the name given to a liquid composition 
which is used very extensively for two purposes first, to act as 
a protective substance to preserve the body on which it has 
been applied from the destructive action of the weather ; second, 
as a decorative agent. 

The first object is brought about by making the paint with 
materials which are not acted upon by the various agents 
present in the atmosphere, such as water, acid vapours, light, 
oxygen, that exert a more or less destructive action on bodies 
which may be exposed to their action. The bodies which have 
been found to resist this destructive action of the atmospheric 
influences are the various so-called drying oils, resinous matters, 
and the pigments. A paint is a liquid composition -which will 
remain liquid until it is applied to the body to be painted, and 
yet when so applied and afterwards exposed to the atmosphere 
will dry and leave behind it a firm, hard (yet elastic), and 
opaque coating, which may be more or less lustrous and be 
capable of resisting the weather. The opacity of the coating is 
obtained by using pigments of various kinds, which also tend to 
increase the resisting power of the paint, and these pigments are 
mixed with liquid bodies, such as oils and spirits, which are 



. 
UNIVERSITY 



8 INTRODUCTORY. 

used partly to obtain a composition that is easy of application, 
and partly to secure volatility, so that when alone or when mixed 
with resinous matters, they will evaporate away and leave 
behind a hard mass firmly binding the pigments to the body 
over which they have been painted. The liquid bodies which 
have been found to answer this purpose best are the drying oils, 
such as linseed oil, which when spread over a surface and 
exposed to the air absorb oxygen and dry into a hard mass; but 
as these oils, for various reasons (which will be more fully dealt 
with later on), cannot be used alone with satisfactory results it 
becomes necessary to mix them with some solvent, such as 
turpentine or shale naphtha, which is volatile. In some kinds 
of paints a little resinous matter is used, which dissolves in the 
solvent; on exposure the latter evaporates off, leaving the resin 
behind in the form of a dry coat on the surface to which it has 
been applied. Paint is always more or less coloured to add to 
the decorative effect. Its primary purpose, however, is to hide 
the character of the surface to which it is applied and, as has 
been pointed out, to protect this surface. 

Varnishes. These bodies are very similar to paints in their 
properties and uses. They differ in giving a transparent 
lustrous coat of a very resistant character to the destructive 
action of the weather. They may be coloured ; but, if so, 
transparent colours are used and not, as in the case of paints, 
opaque pigments. They are composed of a resinous matter 
dissolved in various oils and solvents, the latter forming the 
vehicle by means of which the resin is transferred to the surface 
to be varnished. The special properties of varnishes will be 
dealt with later on. 



CHAPTER IT. 
WHITE PIGMENTS. 

THE white pigments are a very important group of painters* 
" colours," probably the most important, as while the red, blue, 
green, &c., pigments are used simply or almost entirely as 
colouring pigments, the white pigments are used in two ways 
1st, as "body colours," i.e., to give body or covering power to 
paint; 2nd, as "colouring pigments." Thus, in making a red 
paint, white lead or barytes is added to give the necessary body 
and vermilionette is used to colour the paint. On account of 
this dual feature of the white pigments they merit a more detailed! 
account of each individual member of the group than is necessary 
for other pigments. 

The white pigments are a fairly numerous group of bodies 
derived entirely from inorganic sources. Many white bodies are 
known which could be used as pigments, but are not so used on 
account of expense, &c. The following list comprises all that are 
used either on a large or small scale : 

White Lead, basic carbonate of lead, 2 Pb C O 3 Pb H 2 O 2 ; this 
pigment is also sold under a variety of other names. 

Lead Sulphate, Pb S O 4 ; many pigments sold under various 
fancy names consist essentially of this body combined with other 
white pigments. 

Lead Oxychloride, Pb 2 O C1 2 , Pattinson's white lead. 

Zinc White, zinc oxide, Zn O. 

Zinc Sulphide, Zn S ; this body combined with barytes, itc., 
is largely used as a white pigment. 

Barium Sulphate, Ba S O 4 , barytes. 

Barium Carbonate, Ba C O 3 . 

Calcium Sulphate, Ca S O 4 , gypsum. 

Calcium Carbonate, Ca C O 3 , whiting. 

Calcium Oxide, Ca O, quicklime. 

Strontium Sulphate, Sr S O 4 . 

Strontium Carbonate, Sr C O 3 . 

Magnesium Carbonate, Mg C O 3 , magnesite. 

China Clay, hydrated silicate of alumina. 

French Chalk, silicate of magnesia. 



10 WHITE PIGMENTS. 

Of these, the most important are white lead, lead sulphate, zinc 
white, zinc sulphide, barytes, gypsum, calcium carbonate, and 
china clay. 

WHITE LEAD. 

White lead has been known and used as a pigment for cen- 
turies ; the Romans and Greeks used the native carbonate of 
lead or " cerusse, " as it was then called, from which the 
mineralogical name cerussite has arisen. This natural pigment 
is found only in comparatively small quantities, and it is no 
wonder that a process for the artificial production of white lead 
was soon found out and adopted, with the result that natural 
cerussite is not now used as a pigment. 

It is not known when white lead was first made, who made it, 
or to what country it owes its birth. The oldest known method 
is that commonly called the " Dutch method," from the supposi- 
tion that it was invented in Holland ; it is described as the 
Dutch process in an English patent granted in 1787, and there 
is no doubt but that it is the process referred to in three earlier 
patents granted in 1622, 1635, and 1745, in which it is spoken 
of as an old process. Evidently white lead has been made for 
several centuries. During all this period there has been but 
little change made in the Dutch process. But in the interval 
inventors have not been idle, for there is no other pigment 
which has attracted so much attention at their hands as white 
lead ; and the number of processes and modifications of processes 
which have been devised, is almost innumerable. With all this 
invention, the ancient Dutch process still retains its pre-emi- 
nence as the best process for the manufacture of white lead. 

MANUFACTURE OP WHITE LEAD. 

White lead, the basic carbonate of lead, is manufactured by a 
-variety of methods. It is not easy to classify these processes 
into groups, as they not unfrequently pass one into the other 
imperceptibly. The author suggests the following classification, 
which is based on the principles which appear to underlie the 
various methods adopted, or which have been proposed and used 
on a limited scale : 

1st Group. Stack method. 

2nd Group. Chamber methods. 

3rd Group. Precipitation processes based on the action of 
carbonic acid gas on various lead salts. 



STACK METHOD. 



11 



4th Group. Precipitation methods based on the action of 
alkaline carbonates on various lead salts. 

5th Group. Miscellaneous methods. Most of these are now 
obsolete and are only of historical interest. 



1st GROUP. STACK METHOD. 

Only one process is included in this group, the old Dutch or 
stack process. No process which is now in use or which has 
been proposed can claim the antiquity that this process can, and, 
notwithstanding all the labours of chemists and white lead 




2. Shed for making white lead. 



12 



WHITE PIGMENTS. 



makers to supersede it, for reasons which will be pointed out 
presently, it still remains the best process for the manufacture of 
white lead. Tradition assigns its discovery to the Dutch and to 
a person named Stratingh in particular. It must be at least 300 
years old. Since 1787 this process has been carried on without 
much alteration in its details. The Dutch method is used in all 

parts of the world for the manufacture 
of white lead, and there is but little 
variation in the details of the process 
and in the construction of the plant 
used in different countries. 

The plant used in the stack process 
is shown in Figs. 2 to 5. A shed of 
brickwork, Fig. 2, is built, the size of 
which varies a little, but averages 16 
feet long by 13 feet wide and 20 feet 
high ; this may have either a lean-to 
roof, as shown in the figure ; or, as in 
some works, two of these sheds are 
built back to back, with a single-ridge 
roof between them. 

In some places parts of the structure 

are built below the level of the ground, but there is no advan- 
tage to be gained by so doing. A large white-lead works will 
have a number of these sheds, so as to keep the workmen fully 
occupied with filling and emptying them. 

A number of earthenware pots are provided. These pots vary 
in size at different works, but an average size is 8 inches high by 





Fig. 5. 



Fig. 4. 

4 inches in diameter. In shape they resemble crucibles (see Fig. 
3), but have a shelf inside, as shown. In the bottom of these 
pots is placed some weak acetic acid or vinegar; this diluted acid 
contains about 2 to 3 per cent, of actual acetic acid. On the 
shelf inside the pot is placed a roll of thin sheet lead (Fig. 4), 



STACK METHOD. 13 

made from a strip of lead 2 feet long by 4J inches broad. In a 
stack of ordinary dimensions some 11,000 to 12,000 of these pots 
will be used, and they will contain about 800 to 900 gallons of 
weak acid. 

The stack is built up as follows : First, a layer of ashes, upon 
which is placed a layer of spent tan of about 3 feet in thickness. 
In the older Dutch method horse-dung was used, but this is open 
to some disadvantages which will be pointed out presently ; 
the use of tan was introduced in England so that this modifi- 
cation of the Dutch process is sometimes spoken of as the English 
method. This layer of tan is pressed down very firmly and is 
evenly spread ; on it is placed a layer of the pots, which layer is 
kept at a distance of about 6 inches from the sides of the shed. 
In some works the outside rows of pots are made of larger size 
than the others, so as to act as supports for a layer of flooring 
boards. In other places the pots are all of one size and wooden 
supports for the boards are provided. 

On the top of the pots is placed a layer of lead buckles or 
gratings (Fig. 5). These are placed face to face in a layer of 
about 3 to 5 inches thick; above these comes the layer of flooring 
boards, a space of about 6 inches being left between them. On 
the top of the boards another layer of tan, then a layer of pots, 
then a layer of gratings, then another layer of boards, and so on 
until the stack is completely built up. The number of sets 
of layers varies from seven to eleven. The doorway through 
which the filling is done is closed as the work progresses by 
boarding, but a small space is left at the top through which the 
progress of the operation can be observed, and fresh additions of 
material made as required to allow for sinking of the tan, &c. 

The quantity of lead used varies considerably, or from about 
3 tons to 7 tons in a layer of materials, so that in a large stack 
there may be something like 85 tons of lead. 

In stacks of very large area it is usual to construct chimneys 
throughout the mass, whereby the steam which is produced 
during the operation is carried off; in stacks of small area, these 
chimneys are not required, as the space around the side walls of 
the shed affords a sufficient outlet. 

When the stack is built up it is left for a period of about 
three months. During this period the stack gets quite hot 
(140 F.) through the fermentation of the tan which sets in; 
large quantities of carbonic acid gas are given off, and the acetic 
acid is converted into vapour. The "blue lead" is gradually 
converted into "white lead." At the end of three months the 
stack is pulled to pieces. As the boards are removed it is found 



14 WHITE PIGMENTS. 

that the lead which has been corroded still retains the form of 
the blue lead, but is more bulky in volume, is white or greyish 
in tint, and opaque. The corrosions are not of a uniform 
character throughout the whole of the stack ; in some places 
they are porcellaneous and flaky, are firm to handle, do not 
break up, and give the best quality of white lead ; in other 
parts of the stack the corrosions are soft, easily crumble to a 
fine powder or dust when handled, and do not give a good 
quality of white lead. In some places the lead may be dis- 
coloured owing to a variety of causes, such as the presence 
of tarry matter in the acid (especially when crude pyroligneous 
acid is used), by droppings of coloured water from the layer 
of tan on to the lead, &c. In chemical composition the cor- 
rosions will vary ; in some places they will approximate 
closely to the normal composition of white lead, 2 Pb C O 3 , 
Pb H 2 O 2 , in others more nearly to that of 3 Pb C O 3 , Pb H 2 Og, 
while in others they consist of the normal carbonate, Pb C O 3 . 

As the stack is being pulled to pieces the corrosions are 
carried to the grinding rooms. The method of treating the 
white lead varies in different works, but the following may 
be taken as a good example of the usual manner of working : 
The corrosions are first passed through a pair of rolls ; these 
break up the masses, the white lead crumbles to powder, while 
the unchanged blue lead is flattened out into thin sheets. The 
crushed materials are then sieved, which separates the white 
from the blue lead ; the latter is sent to the melting pot where 
it is melted and re-cast for use in building another stack. The 
white lead is sent into tanks full of water, where it is thoroughly 
agitated, and the small traces of acetate of lead which the 
corroded lead contains washed out of it. While still wet the 
white lead is ground as fine as possible under edge runners or 
between rollers, and then dried, when it is ready for sale. As 
the grinding must be thorough, the lead is passed through 
several sets of grinding mills. 

Grinding white lead is a source of danger to the workpeople, 
for the fine dust flies about the room in which it is done and is 
breathed by the workpeople, who, sooner or later, suffer from 
lead poisoning ; much of this danger is avoided by grinding the 
lead in a wet condition only, when the particles of lead are 
practically too heavy to fly about. 

The greatest risk now arises in the packing of the ground lead y 
as the workmen frequently get some on their hands and eat 
their meals without previously washing their hands. Lead 
poisoning may be prevented by drinking water acidulated with 



STACK METHOD. 15 

sulphuric acid, whereby the lead absorbed into the system is 
converted into the harmless sulphate of lead. The great trouble 
is that the workmen will not take sufficient care to make use of 
these precautions. 

It was stated above that in the early or Dutch modification of 
this process, horse-dung was used as the source of the heat and 
carbonic acid necessary to carry on the process ; while with 
dung the process is quicker (only taking from 8 to 9 weeks), yet 
it is not so good as the English method with tan, the product is 
not quite so regular in composition, and it is more liable to 
discolouration owing to the evolution of sulphuretted hydrogen. 
from the decomposing dung, and to its combination with lead to- 
form the objectionable black sulphide. 

The theory of the process of white-lead making by the Dutch 
process, which at present is most favoured by chemists, and was- 
substantiated by some experiments carried out by Hochstetter, is 
due to Liebig. The first action which goes on in the stack is to- 
convert the blue lead into basic acetate of lead ; this is brought 
about by the heat of the fermenting tan, or dung, causing the 
evolution of acetic acid from the liquid in the pots, which, 
attacking the lead, causes the production of the normal acetate 
of lead, thus 

1. Pb + 2HC 2 H 3 2 = Pb2C 2 H 3 2 + H 2 

Lead. Acetic acid. Normal lead acetate. Hydrogen* 

The normal lead acetate, under the influence of water and heat, 
parts with some of its acetic acid and passes into the basic 
acetate, thus 

2. Pb2C 2 H 3 2 + 2H 2 = 2Pb2C 2 H 3 2 ,PbH 2 2 + 2HC 2 H 3 0* 

Normal lead Water. Basic lead acetate. . Acetic acid. 

acetate. 

The acetic acid is ready to attack a further quantity of blue 
lead. The basic acetate is now attacked by the carbonic acid 
evolved by the fermenting tan, the acetic acid it contains is 
liberated and its place taken by the carbonic acid, and white 
lead is formed, thus 



3. 2Pb2C 2 H 3 2 + 2C0 2 = 2PbC0 3 , PbH 2 2 

Normal lead Carbonic White lead. Acetic acid. 

acetate. acid. 

Although the reactions shown in the above equations are 
those usually accepted as representing the formation of white 
lead from blue lead in the stack process, yet they are probably 



16 



WHITE PIGMENTS. 



not quite correct ; the evolution of hydrogen in the first step in 
the process is rather improbable ; a better explanation would be 
the following : (1) By the action of moisture and oxygen on 
the lead there is formed lead hydroxide, thus 

1. Pb + H 2 + O = PbH 2 2 

Lead hydroxide. 

Then this being acted on by the acetic acid forms the normal 
or neutral acetate and water, thus 

2. PbH 2 2 + 2HC 2 H 8 2 = Pb2C 2 H 3 2 + H 2 

The normal acetate now combines with lead hydroxide to 
form basic lead acetate, thus 

3. Pb 2 C 2 H 3 2 + 2 Pb H 2 2 = Pb 2 C 2 H 3 2 , 2 Pb H 2 O 2 

Basic lead acetate. 

This is now acted upon by the carbonic acid with the for- 
mation of white lead and normal acetate, thus 

4. 3[Pb2C 2 H 3 2 , 2PbH 2 2 ] + 4C0 2 = 3Pb2C 2 H 3 O 2 

Normal acetate. 
+ 2 [2 Pb C 3 , Pb H 2 2 ] + 4 H 2 O 

White lead. 

The normaj acetate thus reproduced then forms more tribasic 
acetate by the reaction shown in equation 3. It is again decom- 
posed by the carbonic acid, as shown in equation 4, so that a 
continuous cycle of changes is set up ; the lead being oxidised 
to lead hydroxide, and this converted into white lead, the basic 
carbonate, pari passu with its formation. 

As a rule, nearly all the blue lead is converted into white 
lead, one ton of lead producing one and a quarter ton of white 
lead, the amount varying from time to time according to the 
-degree of perfection with which the corrosion has proceeded. 

The great fault of the Dutch process is the great length of 
time required (8 to 12 weeks), the great amount of capital it 
takes to construct a stack of lead, and the loss of interest which 
takes place on the capital while the lead is in process of making. 
Then there is always a risk, owing to some defect, of producing 
a useless and imperfectly corroded lead, which has to be sent to 
the smelting furnace and again reduced to blue lead. Hence it 
is that inventors have turned their attention to devising other 
methods of producing white lead which shall be free from the 
defects of the Dutch process ; so far, however, no such method 
has been discovered. 



CHAMBER METHODS. 17 

White lead, as made by the process described above, is a basic 
carbonate of lead having the composition 

Lead carbonate, Pb C 3 , .... 68-95 per cent. 
Lead hydroxide, Pb H 2 2 , . . . 31 '05 

100-00 
Lead monoxide, Pb 0, . . . . 86 '32 per cent. 

Carbonic acid, C 2 , 11 '36 

Water, H 2 0, 2 -32 

100-00 
therefore having the formula 2 Pb C O 3 , Pb H 2 O 2 . 

As will be seen hereafter, when the properties of white lead as 
a pigment come to be more fully considered, the carbonate is the 
substance to which white lead owes its colour and body ; while 
the hydroxide with which it is associated, by chemically com- 
bining with the oil used to convert the white lead into a paint, 
imparts to the white lead great covering properties. 

Numerous samples of white lead, of both high and low qualities, 
have been analysed by many chemists, some of which will be 
given later on. These analyses show that when the composition 
of any sample varies greatly from the figures above given, it 
is more or less defective. Generally, an increased proportion of 
carbonate, while causing the colour to be better, reduces the 
covering power; on the other hand, an increase in the amount of 
the hydroxide causes a loss of body and opacity. If the sample 
contain any monoxide then the tint becomes more yellow or 
greyish. Some of these points will be touched upon when con- 
sidering the other processes for the production of white lead. In 
the meantime the success or non-success of any process depends 
upon the approximation of the white lead produced to the 
composition above given. 

2nd GROUP. CHAMBER METHODS. 

In one sense the Dutch method just described is a chamber 
method, but it has been classed as a separate group because 
while being made in a chamber, it differs materially from those 
now to be described. In these processes the operation of white- 
lead making is carried on in large chambers in which metallic 
lead is placed, and into which currents of carbonic acid gas, 
acetic or other acid vapours, are passed, together with air and 
steam. The different methods are distinguished one from another 
by the construction of the chambers, the method of admitting the 

OF THE * 




18 WHITE PIGMENTS. 

acid gases, &c., and in other points. Although many such have 
been invented and will be found described in the Patent Records, 
yet very few are in actual operation for the production of white 
lead ; and of those which have become obsolete very little is 
known beyond the, often very scanty, description which is to be 
found in the specification of the patent which protected the 
process. 

In all the accounts which have appeared describing these pro- 
cesses, and which are evidently copied from one source, the 
chamber process is given as "the German method ;" but it is a 
matter of doubt whether the process was invented in Germany or 
not. The author is inclined to consider it to be of English origin, 
partly because it is described in a patent taken out in 1749 by 
Sir James Creed, who makes no mention of having taken it from 
a foreign source. This was the first patent to describe a chamber 
process, and since then many have been patented ; but the author 
must refer readers to the Patent Records for an account of these 
or to a series of articles which appeared in The Chemical Trade 
Journal in October and November, 1890. 

1. Creed or German Process. This process was the first in 
which chambers were used in conjunction with lead and acid 
gas'es. In detail it is carried out as follows : A chamber of 
brickwork is built of any convenient size and with few openings, 
the usual ones are a door to enter into the chamber for the pur- 
pose of filling it, and an opening in the roof for ventilation ; 
sometimes a window or two for the purpose of observation is- 
added. The chamber has a number of shelves, on which are 
placed sheets or gratings of lead ; it is immaterial which are 
adopted, although the gratings expose more surface to the action 
of the various gases which are used. When all the lead is placed 
on the shelves, the doors are closed, and currents of aqueous, 
vapour, air, carbonic acid, and acetic acid at once admitted into- 
the chamber. After a period varying from four to five weeks,. 
the white lead will have been formed ; it is collected and treated 
as in the Dutch process. The chemical action which proceeds is 
supposed to be the same as that which takes place in the old 
stack method. The acetic acid acts upon the lead, forming 
neutral acetate of lead ; this, under the action of the aqueous 
vapour, is transformed into basic acetate of lead, and this, in its- 
turn, is changed by the carbonic acid into basic carbonate of lead 
or white lead. 

The quality of the product is usually very good, not, perhaps, 
quite equal to that produced by the Dutch method, but better 
than that produced by the precipitation processes. It is, how- 



CREED OR GERMAN PROCESS. 19 

ever, inclined to be very variable, and the process requires some 
experience to carry out in the best possible manner to ensuie 
a good product. It is desirable, as far as possible, to cause 
the white lead to approximate in composition to the formula 
2 Pb C O 3 , Pb H 2 O 2 , and to do this it is necessary that the gases 
should be sent into the chamber in the proper proportions. If 
excess of acetic acid is present, too much acetate of lead is 
formed, which is not decomposed by the aqueous vapour and the 
carbonic acid ; too much of the latter tends to cause the formation 
of an excess of lead carbonate, and the white lead loses its 
covering powers. On the other hand, too much steam will lead 
to the formation of oxide, especially if the temperature be 
allowed to get high ; the oxide so formed being of a yellow tint 
spoils the colour of the white lead. The same result is brought 
about by a deficiency of acetic acid. Experience is the only 
factor which can guide the white-lead maker in adjusting the 
various gases in the proper proportions. 

The following are some analyses of white leads made by this 
process quoted by Weise : 

1. 2. 3. 4. 5. 

Lead monoxide, 86 '80 86 '24 86 '03 84'69 83 '47 

Carbonic acid, 11-16 1T68 12'28 14*10 16'15 

Water, 2 '00 1'61 1'68 0'93 0'25 

1. Firsts, of the best quality ; good both in colour and body. 

2. Seconds, not so good as No. 1, but still very serviceable 
as a pigment. 

3. Thirds is only just usable as a pigment. 

4. Is not usable except for very common purposes. 

5. Not usable at all ; it contains too much carbonate, and is 
sent to the smelting furnace. 

Various alterations in the details of this method have been 
made from time to time by various inventors, some of which may 
be briefly noted. Burton placed the lead in coils on the shelves of 
the chamber, and passed the current of steam through perforated 
pipes, thereby converting the lead into oxide ; when a sufficient 
amount of this has been formed the current of steam is stopped, 
and a current of acetic acid vapour sent in; this, acting on the 
basic acetate converts it into the basic acetate; when this action 
is finished the acetic acid current is stopped, and carbonic acid 
gas sent in, which acts on the basic acetate and changes it into 
basic carbonate or white lead. These currents of steam, acetic 
acid vapour, and carbonic acid gas are sent in successively until 
all the lead is converted into white, lead, then the currents are 



20 WHITE PIGMENTS. 

stopped, and the white lead is collected and finished in the 
usual way. 

In Richardson's method the lead is soaked in a solution of 
acetate of lead, the action, both of the acetate and the gaseous 
bodies, to which the soaked lead is subjected being facilitated by 
the lead being cast into a granular form. After being soaked, 
the lead is placed on shelves in the chamber, and then subjected 
to the combined action of steam and carbonic acid gas, the 
chamber being maintained at a temperature of about 100 F. ; 
the process is continued until all the lead is converted into 
white lead. 

2. Hatfleld Process. This process resembles those just 
described to some extent, but differs in a few minor particulars. 

The chamber is built with a double wall, and the bottom is 
hopper-shaped. The lead is cast into gratings of the same shape 
.as used in the Dutch process, and placed in trays on shelves in 
the chamber. Into the chamber is sent water and acetic acid in 
the form of spray, at the same time the chamber is maintained 
at a suitable temperature by means of steam pipes. The action 
of the water and acid is to convert the lead into basic acetate of 
lead ; when this has been properly formed the water and acetic 
acid spray is stopped, and a current of carbonic acid gas sent in 
to form white lead as described above. 

3. Thompson's method, patented in 1873 and 1877, was 
worked by the Innocuous White Lead Co., of London. In this 
process the chamber is built of brick, with a large door and one 
or two windows, so that the progress of the operation can be 
observed. The bottom of the chamber is made like a trough and 
acid proof, glass being recommended as a material for its con- 
struction. The roof is built double, so that any liquid which is 
condensed will flow down to the sides and not drop on to the 
corroded lead below. The lead used is cast into gratings which 
are placed on open trucks fitted with shelves ; the lead is soaked 
in a solution of acetate of lead and then wheeled into the 
chamber. A quantity of acetic acid is placed in the bottom 
of the chamber and vapourised by means of steam pipes passing 
through it, by which means the lead is converted into the basic 
acetate ; when this action is complete, carbonic acid gas is sent 
in to change it into white lead. This was said to be of good 
quality. In the practical application of this process, much 
depends upon the temperature at which the chamber is main- 
tained during the operation. If too high, then there is a 
tendency to form oxide of lead which does not readily change 
into white lead ; if too low, then the action of the acid on the 



GARDNER'S ELECTRIC PROCESS. 21 

lead is not energetic enough ; if too much acetic acid is used, 
then the tendency is to form normal acetate of lead which 
reduces the yield of white lead, and at the same time tends to 
cause this to have too much carbonate in its composition. 

A very similar process to this was patented by Morris. In 
this the lead was used in the form of sponge or wire so as to 
expose as much surface as possible to the action of the various 
gases. The acetic acid was placed in vessels on the floor of the 
chamber and steam and carbonic acid gas passed in, a constant 
current of these gases being maintained. The white lead was 
gradually formed, and when complete was collected and finished 
in the usual way. 

4. Gardner's Electric Process. In 1882, Prof. E. V. 
Gardner patented a process for making white lead which, as 
he considers that electricity plays a part, he calls an " electric 
process." 

The specification of this patent (No. 731, of 1882) is very full, 
and is well worth reading by white-lead makers. In the 
specification the conditions most favourable to a successful 
production of white lead are fully stated, and from it the 
following is abstracted : 

As has been previously pointed out, in making white lead by 
the chamber methods there are several factors which require 
attention, if the product is to be a good one. Prof. Gardner 
states these to be as follows : The proper formation of what he 
calls the sub-acetate or sub-nitrate of lead ; these basic salts are 
the compounds of the normal salts with the hydroxide of lead, 
and, therefore, have the formula Pb 2 C 2 H 3 O 2 , Pb H 2 O 2 for 
the sub-acetate, and Pb 2 N 3 , Pb H 2 O 2 for the sub-nitrate. 
It is usual to consider the so-called subsalts of lead as com- 
pounds of the normal salts with the monoxide ; probably both 
kinds of salts exist that is, there are compounds both of the 
monoxide and of the hydroxide of lead with the normal salts of 
lead. In white-lead making it is reasonable to suppose that 
better results would be obtained if the hydroxide compounds 
were formed than if the monoxide compounds were obtained in 
the process of making. Hence the conditions most favourable 
for the formation of the hydroxide should be carefully ascer- 
tained. 

Temperature is an important factor ; this should be from 120 
to 130 F. A lower temperature increases the length of time 
required for the formation of the subsalts, and so increases the 
cost of the process, while the quality of the white is deteriorated, 
owing to its deficiency in hydroxide. Too high a temperature 



22 WHITE PIGMENTS. 

must be avoided, for although a high temperature increases the 
rapidity with which the subsalts are formed, yet it is liable to 
cause them to lose their water of hydration and to pass into the 
monoxide subsalts ; the presence of these in the white lead 
makes it of bad colour and hence deteriorates the quality. Too 
little air, acetic acid, and aqueous vapour also tends to prevent 
the proper formation of the subsalts and, consequently, of the 
white lead of the best quality ; too much acetic acid converts the 
subsalts into the normal salts and, as is well known, these do 
not produce white lead of good quality ; besides which, being 
soluble in water, they are washed off the surface of the lead by 
the aqueous vapour which condenses on the lead, and are thus 
lost for the purpose of making white lead. These are a few of 
the principal conditions which Prof. Gardner points out as being 
necessary for the proper production of white lead of good quality, 
and, although given in connection with his own process, yet 
there is no doubt but that they are applicable to all chamber- 
processes and also to some other methods of making white lead. 

The electric process is carried out in a chamber made of any 
convenient form and material ; it is necessary, however, that it 
should be so constructed that the progress of the operation is 
readily visible. In this chamber are arranged a number of 
shelves covered with tin, a metal which is electro-negative to 
lead. Carbon, or any metal which is electro-negative to lead, 
may be used, but the inventor prefers tin. These shelves are 
connected together in succession by means of strips of tin, so 
that when lead is placed on them they form an electric couple. 
Instead of having the shelves a fixture in the chamber, they 
may be constructed on an open framework fitted with wheels ; 
on this, while outside the chamber, the lead gratings are 
arranged, and when the shelves are full the frame and its 
contents are run into the chamber, but before doing so the 
lead is soaked in a solution of acetate or nitrate of lead. The 
temperature of the chamber is maintained at 120 F. by means 
of steam which is sent into it for that purpose. At the same 
time currents of acetic or nitric acid vapours, made by boiling 
dilute solutions of those acids, are conveyed into the chamber, 
or dioxide of nitrogen with acetic acid may be used. The 
atmosphere of the chamber must be in a misty condition, and 
this is brought about by regulating the current of acid vapours 
and steam ; this state of affairs is kept up for 48 hours, when a 
current of pure carbonic acid gas is sent in for 2 hours, then 
stopped, and the acid gases sent in by themselves for 4 hours, 
when the admission of carbonic acid is again resumed for 



PRECIPITATION PROCESSES. 23 

2 hours ; these alternation of currents of acid gases and steam 
for 4 hours, and acid gases and steam and carbonic acid for 
2 hours, is carried -on for 14 to 15 days, when the whole of the 
lead will be found to be converted into white lead. During all 
this time the temperature must be kept at about 120 R, and 
the atmosphere of the chamber misty ; it is for the purpose of 
closely watching the progress of the process that the chamber is 
fitted with windows. 

The carbonic acid gas may be prepared by any well-known 
method, but the inventor prefers to use a petroleum lamp as the 
source of it. 

After the operation of making the white lead is finished, the 
material is not immediately removed from the chamber, but the 
acid gases are stopped, and steam only sent in, which serves to 
wash the product ; then, after a time, the current of steam is 
stopped, and air only admitted, when the white lead becomes 
dry. It is now taken out of the chamber and finished in the 
usual way. 

The product obtained by Gardner's process is of good colou* 
and body, and closely approaches, if, indeed, it is not equal to, 
Dutch white lead in its properties. One advantage said to be 
possessed by this process over the stack method, is that while 
in the latter it is essential to work with the purest lead which 
can be made, in the new process ordinary commercial lead gives 
excellent results. 



3rd GROUP. PRECIPITATION PROCESSES. 

When a current of carbonic acid gas is passed through a solu- 
tion of a basic salt of lead, such as the basic acetate or the basic 
nitrate, a white precipitate will be obtained, which is due to 
the combination of the carbonic acid with the excess of lead 
oxide contained in the basic salt ; this precipitate consists of a 
more or less basic carbonate of lead. At the same time, a solu- 
tion of the normal salt is obtained, because carbonic acid is too 
weak to displace any other acid from its combination with lead. 
This action of the carbonic acid gas is shown in the following 
equation : 

3 [Pb 2 C 2 H 3 2 , 2 Ph H 2 2 ] + 4 C 2 = 2 [2 Pb C 3 , Pb H 2 2 ] 

Basic acetate of lead. White lead. 

+ Pb2C 2 H 3 2 + 4H 2 

Normal 
acetate of lead. 



24 WHITE PIGMENTS. 

although this may not accurately represent the action which goes 
on in the majority of cases. 

Various salts of lead are used. The differences between the 
various processes based on the principle just described, depend 
upon the kind of salt used, and the method of carrying out the 
operation. 

These processes were introduced in the early part of the 
century, the first patent being dated 1808, and granted to 
E. Noble. The process described consisted in passing a current 
of carbonic acid through a solution of lead acetate. A very 
similar method is known as Thenard's, or the French, process, 
and will be found described below ; while another precipitation- 
method is known as the Kremnitz process, having been largely 
used there for the preparation of white lead. 

The precipitation-processes based on the action of carbonic acid 
gas upon lead salts may be divided into two sub-groups : 
3a. Dry methods, in which the lead salt is used in the dry state, 
or, at the most, simply moistened. 3b. Wet methods, in which, 
the lead is used in the form of a solution. 

3a. DRY PRECIPITATION PROCESSES. 

1. Kremnitz Process. This process owes its name to having 
been worked at Kremnitz in Germany. It is carried on in a 
chamber built of brick or wood, having a number of shelves, on 
which is placed trays containing a paste made of litharge and 
either acetic acid or lead acetate, usually in the proportions of 
100 Ibs. of litharge to 18 pints of acetic acid, or an equivalent 
quantity of lead acetate solution. When the chamber is filled 
carbonic acid gas is sent into it, this becomes absorbed by the 
lead oxide present in the paste, the absorption of the gas being 
facilitated by raking over the paste from time to time, the mass 
being kept moist, as this increases the absorption of the gas. 
The mass originally has a yellowish-grey colour, but as the 
operation progresses it gradually changes into a white ; and 
when all traces of yellow have disappeared, the operation is 
stopped, and the white lead which is made is first washed with 
water, then ground and dried. 

Care is taken not to pass the carbonic acid in too long, because 
this would induce the formation of the normal, instead of the 
basic, carbonate, which means poor white lead. When carefully 
worked, good results can be obtained by this process. 

The following analysis, presumably of a Kremnitz white lead, 
is given in Wagner's Technologic : 



THEXARD PROCKSS. 25 

Lead oxide, . . . . 83 '77 per cent. 
Carbonic acid, . 15 '06 

Water, 1-Q1 



or 



Lead hydroxide, . . . 8 21 per cent. 

Lead carbonate, . . . 91 '21 ,, 
Moisture, 0*42 

which shows that this sample did not approach Dutch white 
lead in composition, but contained more carbonate. 

2. Mullin Process. In this process, which is not now in 
use, litharge was ground into a paste with water; the paste 
was then placed in shallow lead-lined boxes, in layers of about 
an inch and a quarter thick, the boxes were closed by a lid, 
and then into them was sent currents of carbonic acid and 
acetic acid gases ; the litharge was gradually converted inta 
white lead. The process was, presumably, not a successful one, 
or it would not have gone out of use. 

3b. WET PRECIPITATION PROCESSES. 

In this group of processes for the preparation of white lead y 
the lead is used in the form of solution, and the precipitation is. 
effected by means of a current of carbonic acid gas. There 
are a large number of these processes, and many are still in 
use on the large scale. The differences between the various, 
processes belonging to this group depend upon a variety of 
circumstances, such as the method of preparing the solution 
of lead, and the form of apparatus used, on which to a large 
extent depends the subsidiary, but not unimportant point, the. 
method of applying the carbonic acid to the lead solution. 

1. Thenard Process. This process, from having been worked 
on a large scale at Clichy, in France, is known as the French 
process ; it is also described in the patent granted to E. Noble- 
in 1808. 

The principle of the Thenard process, which is also applicable 
to many others of this group, is that when a solution of normal 
lead acetate is boiled with litharge, some of the latter is dis- 
solved, and a solution of basic lead acetate, known as "Goulard's 
Extract," "Extract of Saturn," &c., is obtained. The reaction 
is expressed in the following equation : 

Pb 2 C 2 H 3 2 + 2 Pb + 2 H 2 = Pb 2 C 2 H 3 2 , 2 Pb H 2 2 

Normal acetate Litharge. Water. Basic acetate oflead. 




26 WHITE PIGMENTS. 

If a current of carbonic acid is passed through this solution of 
basic acetate of lead, the lead hydroxide it contains is precipitated 
as a more or less basic carbonate, thus 

3 [Pb 2 C 2 H 3 O, 2 Pb H 2 2 ] + 4 C 2 = 2 [Pb C 2 , Pb H 2 2 ] 

Basic acetate of lead. Carbonic acid. White lead. 

+ 3Pb2C 2 H 3 2 + 4H 2 

Normal acetate 
of lead. 

The normal acetate which is thus re-formed can be used again 
for preparing a fresh solution of basic acetate of lead \ of course, 
while, theoretically, a very little normal acetate is sufficient for 
the preparation of a large quantity of white lead, and there 
should be no loss, practically, a small quantity of new acetate 
has to be added from time to time to make up for the little loss 
which does occur. 

The apparatus used in carrying out the French process at 
Clichy is shown in Fig. 6. In a vessel, A, of convenient size, 
litharge is dissolved in a solution of lead acetate, the solution 
being accelerated by heating the solution by means of the steam 
pipe, B ; from this vessel the liquor in A runs into another 
vessel, C, in which all insoluble matter settles out. The clear 
solution is now run into a trough-shaped vessel, D, into which 
dip a number of pipes connected with the large main pipe, E, 
through which a stream of carbonic acid gas from the generating 
system, F G, flows. This system consists of an oven, F, in 
which is burnt a mixture of chalk and coke, from which a large 
quantity of carbonic acid gas is evolved ; this gas is washed in 
the apparatus, G, by passing it through water, after which it 
passes into the solution of lead in the vessel, E, precipitating 
white lead from it in so doing ; the length of time of treating 
depends upon the quantity and basicity of the lead solution, but 
usually it takes from 12 to 14 hours. At the end of this time 
the current of gas is stopped and the white lead allowed to 
settle ; the clear liquor, which is a solution of the neutral 
acetate, is run into a vessel, H, from which it is pumped up by 
the pump, I, into the vessel, A, to dissolve more litharge for a 
fresh operation. The mass of white lead which settles at the 
bottom of the vessel, E, is run into another vessel, K, from 
whence it passes on to filters to be washed, and then it is finished 
in the usual way. 

The product given by this process is fairly good, but liable to 
vary in composition from time to time, according to the strength 
of the solution of basic lead acetate, and to the basicity or pro- 



THENARD PROCESS. 



27 




28 



WHITE PIGMENTS. 



portion of lead oxide the lead acetate has dissolved. These are 
points to which reference will be made in describing other 
processes. 

2. Cory Process. The same materials are used in this pro- 
cess as in the last, viz., basic lead acetate and carbonic acid gas, 
but it differs in the form of apparatus used. The process has 
been worked on a large scale for a long period. It was first 



Sect/of i offlief'/iamberm wJtt'ch 
tie Solutions of Lead aw wnvert- 
ed Mo Gu'bo/M/e or lead. 




Fig. 7. Cory's process for making white lead. 

patented in 1838, and the white lead produced by it is favour- 
ably spoken of by users. The author believes that the process 
is still in use. 

The plant used is shown in Fig. 7. A chamber is built of 
brickwork ; the bottom is made watertight and sloping towards 
one end so that any liquor which may fall upon it drains away 
into a tank ; this chamber is divided by a number of vertical 



MILKER PROCESS. 29 

partitions into compartments ; the partitions are so constructed 
that each alternate one does not quite reach the top while the 
others do not quite reach the bottom, as shown in the figure ; 
the object of this is to make the carbonic acid gas, which is sent 
into the chamber at one end, take a circuitous course before it 
passes out at the other end. Above the chamber is a tank, the 
bottom of which forms the roof of the chamber, which bottom is 
perforated with a large number of fine holes, so that any liquor 
which may be run into the tank flows through into the chamber 
below, in a fine stream like rain. In another tank a solution of 
basic acetate of lead is prepared in the usual way, this flows into 
the chamber tank and from thence into the chamber ; here it 
comes into contact with carbonic acid gas which is sent into the 
chamber, the action between the lead solution and the gas being 
facilitated by the liquor being in such a finely divided form. 

The lead solution falls down to the bottom of the chamber, 
and thence into settling tanks, where the white lead which is 
formed settles ; it is collected, washed, dried, and finished in the 
usual way, while the solution of neutral acetate of lead, which is 
also obtained, is used over again. 

3. Milner Process. Milner does not use the basic acetate of 
lead in his process, but prepares his lead solution by taking 4 Ibs. 
of finely-ground litharge, and mixing it with 1 Ib. of salt dis- 
solved in 16 Ibs. of water, the mixture being made in wooden 
tanks. The patentee states that these should be made of yellow 
pine ; oak-wood tanks will not do. In the tanks the mixture is 
well agitated for about 4^ hours, at the end of which time it will 
have been converted into the basic chloride of lead. When the 
basic chloride has been fully formed, it is run into covered 
wooden tanks fitted with agitators ; through these tanks a 
current of carbonic acid gas passes, which, acting on the basic 
chloride, converts the latter into white lead. Instead of this 
procedure, the basic chloride may be mixed in lead-lined tanks 
with caustic soda, and gas is passed into the tanks, as before, 
until the liquor ceases to be alkaline. This point is ascertained 
by the workmen taking a little of the mixture out of the tanks 
from time to time ; if it appear viscid, forming a homogeneous 
mass and an even layer on the sides of the glass, then sufficient 
gas has not been passed in; if, however, it forms a sort of 
arborescent pattern on the sides of the glass, the operation is 
finished ; the current of gas is then stopped, and the white lead 
sent to be finished in the usual way. 

The process is said to yield a white lead of good colour and 
body, and very heavy, weighing about 200 Ibs. to a cubic foot. 



30 WHITE PIGMENTS. 

It was worked by the Sankey White Lead Co., but has been dis- 
continued for some time. 

4. Martin Process. Martin's process for the preparation 
of white lead is based on the action of carbonic acid on solutions 
of basic acetate of lead ; whether this process was ever used on 
the large scale the author has no knowledge. One great fault of 
all precipitation-processes for the manufacture of white lead is 
that they are apt to give a product which is more or less crystal- 
line, a condition fatal to its being of good quality ; the colour 
may be good, but the body is always deficient. The patentee 
states that this depends upon the proportion of acid solvent of the 
litharge to the water which is used in the process ; if the water 
be in excess, then too much basic salt is formed, and the carbonic 
acid, acting too energetically upon this, causes the formation of a 
crystalline product; therefore the acid solvent must be in excess. 
Martin prepares a solution of the neutral acetate of lead in one 
and a-half times its weight of water, or litharge may be dissolved 
in acetic acid in such a way as to produce a similar solution. 
3,600 gallons of this solution are placed in a tank fitted with an 
agitator; there is then added 4 to 6 tons of granulated lead, and 
half a ton of litharge. After thoroughly mixing the materials 
together, carbonic acid gas is passed in for an hour, when all the 
litharge will have been converted into white lead, then half a ton 
more litharge is added, and more carbonic acid ; in about an 
hour this second lot of litharge will be converted into white 
lead, then more litharge is added, carbonic acid being mean- 
while sent in ; these additions of litharge are continued hourly 
until sufficient white lead has been formed, when it is collected 
and finished in the usual way. If thought desirable, instead of 
adding the litharge in lots every hour it may be run in in a 
constant stream. During the operation the temperature is 
maintained at about 100 F. 

The distinctive feature of this process is using the litharge 
in an undissolved form, and strong solutions of lead acetate. 
In the absence of practical experience of the process it is not 
easy to speak definitely on the effect of using such strong 
solutions; but, judging from the known effects of using strong 
solutions on the character of precipitates obtained in other cases, 
one would naturally imagine that the white lead formed would 
have a crystalline character, and not that amorphous condition 
which is required in good white lead, still the patentee states 
that such is not the case. 

When the principles which underlie these precipitation- 
processes are considered, it becomes evident that the character 



FOURMEXTIN PROCESS. 31 

of the white lead, both chemically and physically, materially 
affects its value as a pigment. This character will depend 
upon the character of the solution of lead which is used, the 
temperature at which the reaction between the carbonic acid 
gas and the lead salt takes place, and the strength of the 
solutions used ; on these points information is scanty, and very 
few of the inventors of white-lead processes have mentioned 
the influence of any of them. The character and basicity of 
the lead salt will have some influence on the result ; the- 
basicity should be due to the presence of lead hydroxide, 
and not to lead oxide, or, at all events, the latter should be 
present in only small quantities. To ensure the production 
of lead hydroxide, water seems to be necessary, and therefore 
should be used in sufficient quantity. The quantity of carbonic 
acid should be so regulated that not more than two-thirds of 
the base present is converted into carbonate ; if too much gas 
is used, then all the base will be liable to be converted into- 
carbonate, and the white lead has a tendency to become 
crystalline ; the difficulty is to ascertain when sufficient gas- 
has been used. The strength of the solution of lead will also- 
have some influence, but the diversity of opinion among white- 
lead makers as to the proper strength is great; some prefer 
strong solutions, others weak ones. As a rule, weak solutions, 
give the finest precipitates, and strong solutions give the coarsest. 
The temperature at which the operation is conducted will have 
some little influence; cold solutions will cause the formation 
of fine precipitates, while hot solutions tend to give rise to crys- 
talline precipitates, due to the fact that the reaction between 
the carbonic acid and the lead salt takes place too readily ; still 
it is not desirable to work with solutions that are too cold; 
the best temperature is from 100 to 120 F. 

5. Fourmentin Process. This was proposed many years- 
ago, and somewhat resembles Milner's process. Litharge is 
taken and treated with salt in such proportions as to convert 
it into oxychloride of lead ; this body is placed along with 
water in a number of cylindrical vessels fitted with radial 
beaters. Carbonic acid is sent in, while the temperature is 
maintained at the boiling point. When the reaction between 
the acid and the lead has finished, the current of gas is stopped 
and the product run into a boiler, in which it is boiled with a 
quantity of finely-powdered carbonate of lime, equivalent to the 
amount of salt used in the preparation of the oxychloride, this 
boiling being continued until, on taking out a sample, and 
filtering off and testing the clear liquor with ammonia and 



32 WHITE PIGMENTS. 

ammonium sulphide, no precipitate forms ; the period of boiling 
varies from two to four hours. When the boiling with the 
carbonate of lime has been continued long enough, the opera- 
tion is stopped, and the white lead allowed to settle out, collected 
and finished in the usual way. 

6. Spence Process. The principle of this process consists in 
boiling a salt of lead (the oxide or carbonate gives the best 
results, but the sulphate or other salt which can be dissolved 
by caustic soda may be iised) with a solution of caustic soda 
until the alkali is saturated with lead ; then a current of 
carbonic acid is passed through the liquor, and white lead is 
precipitated, while carbonate of soda is formed. The latter can 
be causticised by means of lime, and used over again. The 
white lead which is precipitated is collected, washed, and finished 
in the ordinary way. This process has not been used on a 
commercial scale. 

7. Maclvor Process. The principle of this process depends 
upon the fact that when litharge is acted upon by acetate of 
ammonia under the combined influence of heat and pressure, it is 
converted into basic acetate of ammonia and lead, while ammonia 
is liberated in the free condition, and dissolves in the water 
which is present to form the liquor ammonia of commerce. 
Then, when a current of carbonic acid gas is passed through the 
mixture of basic acetate of lead and ammonia, the lead is precipi- 
tated as basic carbonate or white lead, of good colour and 
covering power ; while acetate of ammonia is re-formed and can 
be used again for dissolving a fresh batch of litharge. The pro- 
cess is carried out somewhat in the following manner : Into a 
digester made of strong iron plate lined with lead, is placed a 
solution of acetate of ammonia of not less than 5 per cent, 
strength and a quantity of litharge which has been previously 
very finely ground. The proportions of the two will depend 
upon the strength of the solution of ammonia acetate which is 
used ; for that given, 1 ton of litharge is used for 1,200 gallons of 
liquor. The digestor is closed. The acetate solution is sent 
through a heater so that it may have a temperature of from 60 to 
100 p., and then into the digestor, passing into it from a pipe 
fitted with a conical spreader at its lower end ; the acetate solu- 
tion flows upwards through the litharge, effectually agitating the 
mass and so assisting its solution ; from the digestor the liquor is 
drawn off from the upper portion by means of a pump and passed 
through the heater and again into the digestor, this cycle of flow 
being continued until all, or nearly all, the litharge is dissolved. 
The solution of basic acetate of lead and ammonia is now passed 



PRECIPITATION PROCESSES WITH ALKALINE CARBONATES. 33 

through a filter-press into a cooler, from which it flows into a 
carbonator. The cooling of the liquor causes the separation of 
much of the basic acetate of lead in the form of fine crystals, so 
that in the carbonator a fine magma is presented to the action of 
the carbonic acid gas, which is sent into it from any convenient 
source. A circulation of the mass in the carbonator is kept up 
by drawing off from the upper portion of the carbonator and 
forcing it by means of a pump through a pipe, with a conical 
spreader at its end, to the bottom of the carbonator ; in this way 
every part of the mass of liquor and crystals is made to come in 
contact with carbonic acid gas. The white lead is rapidly formed 
as a fine white precipitate. When it is considered that the 
carbonation is finished the whole mass is passed through a filter- 
press, so as to separate the white lead formed, while the liquor, 
which consists of a solution of acetate of ammonia, together with 
unchanged basic acetate of lead and free ammonia, is sent to the 
digestor to be used again. The process is a rapid one ; the solu- 
tion of the litharge in the acetate of ammonia does not take long, 
while the conversion of the basic acetate of lead into white lead 
in the carbonator is almost instantaneous. The process is being 
worked by a limited company. 

4th GROUP. PRECIPITATION PROCESSES WITH 
ALKALINE CARBONATES. 

When a solution of sodium carbonate, or other alkaline car- 
bonate, is added to a solution of lead, a white precipitate of a more 
or less basic carbonate of lead is obtained ; insoluble basic or 
neutral salts of lead, such as the oxy chloride or the sulphate, are 
also acted upon by alkaline carbonates, and basic lead carbonate 
is formed; these reactions are formulated in the following 
equations : 

Pb2C 2 H 3 2 , 2PbH 2 2 + 2Na 2 C0 3 = 2 Pb C 3 , Pb H 2 2 

Basic acetate of lead. Sodium carbonate. White lead. 

+ 2NaC 2 H 3 2 + 2NaOH 

Sodium acetate. 

Pb 2 OCl 2 + 2Na 2 C0 3 + H 2 = 2 Pb C 3 + 2NaCl + 2NaOH 

Lead Sodium Water. Lead Sodium Sodium 

oxychloride. carbonate. carbonate. chloride. hydroxide. 



PbS0 4 + Na 2 C0 3 = PbC0 3 + 

Lead sulphate. Sodium Lead Sodium 

carbonate. carbonate. sulphate. 

The chief difficulties met with in carrying out the processes 



34 WHITE PIGMENTS. 

depending upon the action illustrated in the above equations, are 
to prevent the formation of a highly crystalline neutral carbonate 
and to ensure that the precipitate shall have the necessary 
amount of basicity ; for, as will be seen from the above equations, 
the tendency is to form the normal carbonate of lead instead of 
the basic carbonate. 

These methods of preparing white lead early attracted atten- 
tion from white-lead makers, and many and various have been 
the processes which have been patented and tried for the manu- 
facture of the pigment by such methods. While there is no- 
doubt that good white lead can be made by them, yet the results 
seem to be so variable that from a commercial point of view 
these processes have always been failures. 

The first patented process belonging to this group dates from 
1797 when the Earl of Dundonald secured a patent for making 
white lead from the oxychloride of lead. 

1. Dundonald Process. Litharge is taken and is treated 
with sufficient salt and water as to convert it into the oxy- 
chloride of lead, in the manner which will be found more fully 
detailed on p. 29. The insoluble oxychloride is collected, washed 
to free it from alkaline salts, then boiled in a solution of potash 
(potassium carbonate), when it is converted into white lead,, 
which is collected, and, after washing, dried ; it is then ready for 
use. A very similar process was patented some years later by 
James Kier. No record exists as to whether this process was. 
much, if at all, used on the large scale. 

2. Pattinson Process. Mr. Hugh Lee Pattinson, a large 
lead-smelter of Newcastle, has prepared white lead by many 
processes ; " Pattinson's white lead " (which see) is the oxy- 
chloride of lead. The process, which comes under the present 
group, has for its object the preparation of ordinary white lead. 
Chloride of lead prepared by any convenient process is mixed 
with carbonate of lime in the proportion of their chemical 
equivalents, 278 to 100, and the mixture is ground with water 
for several hours, then allowed to stand all night, the clear 
liquor (which consists principally of a solution of chloride of 
calcium) run off, more water added, and the grinding resumed 
for a few hours ; then it is again allowed to stand all night and 
the clear liquid again drawn off. These operations are continued 
until the effluent water is tasteless. The white lead, after 
being finished in the usual manner, is ready for use. Instead 
of the process just described a solution of the carbonate of lime 
or of carbonate of magnesia, made by means of carbonic acid, is 
used to act on the lead chloride. 



LOWE PROCESS. 35 

In another modification of the process, chloride of lead and 
carbonate of calcium are placed in a revolving cylinder and a 
current of carbonic acid gas sent into the mixture, preferably 
the gas is used at a pressure of four or five atmospheres ; after 
four days the aqueous liquor, which is, as before, a solution of 
calcium chloride, is drawn off, more water run in and the gas 
again passed in for two clays longer, when the reaction is 
completed, and the white lead only requires finishing to be 
ready for use. This process does not seem to have been much 
used. 

3. Dale and Milner Process. The inventors take litharge 
or a basic salt of lead and grind it with water and bicarbonate 
of soda for some time, when white lead is formed. This process 
was worked on a large scale for a short time, but it was super- 
seded by Milner's process described above. A process patented 
by Isham Bagys was almost identical with this. The results 
were not very satisfactory as the white lead obtained was rather 
too crystalline in structure. 

4. Watt and Tebbutt Process. This consisted in treating 
sulphate of lead, first with lime, then with potash. Cooper uses 
25 Ibs. of sulphate of lead to 10 Ibs. of potash. The action of 
alkaline carbonates upon lead sulphate is, at the best, but im- 
perfect, and a complete change into carbonate is never obtained. 

5. Delafield Process. Delafield uses nitrate of lead, which 
he prepares by dissolving one cwt. of litharge in one cwt. of 
nitric acid and just enough water to form a saturated solution. 
This is heated by steam to a temperature of about 200 F. 
When a hot solution of 70 Ibs. of potash is run in, white lead is 
precipitated, which is collected and, after washing, dried. The 
product is liable to contain too much carbonate and, therefore, 
to be deficient in body. 

6. Rowan Process. This resembles the "Watt and Tebbutt 
process, only the action between the lead salts and the alkaline 
carbonate is effected under a pressure of from 30 to 40 Ibs. 

7. Lowe Process. In Patent No. 9,122 of 1887, a process for 
making white lead is described, which consists of the following 
operations : 50 Ibs. of lead acetate, or 43 -6 Ibs. of lead nitrate, 
are dissolved in 25 to 30 gallons of water ; to this solution 
is then added 23 Ibs. of solid bicarbonate of soda or 26-4 Ibs. of 
solid bicarbonate of potash, when a precipitate of a more or less 
basic carbonate of lead will be obtained. In another vessel 
25 Ibs. of lead acetate and 15 Ibs. of litharge are digested with 
12 J gallons of water for 8 to 10 hours, when the product which 
is obtained is mixed with the precipitate obtained in the first 



36 WHITE PIGMENTS. 

instance. White lead is formed and is collected and finished in 
the common way. An analysis of a sample of white lead made 
by this process is given in the patent, as follows : 

Lead monoxide, Pb 0, .... 86 '185 per cent. 
Carbonic acid, C 2 , . . . . 11'270 ,, 
Water, H 2 0, 2 -545 

which differs but little from that of the Dutch process white 
lead. 

8. Condy Process. This process was patented in 1881, and 
has been worked on a large scale ; but whether the process is 
now in use or not the author is unaware. In this process, acetic 
acid of 1 -045 specific gravity is diluted with about five times its 
volume of water, and allowed to act on granulated lead until a 
solution of lead acetate of 1 -2 specific gravity is obtained ; this 
solution is evaporated to dryness, when the bibasic lead acetate 
is obtained. 275 Ibs. of bibasic acetate of lead, 112 Ibs. of 
litharge and 5 gallons of water are ground together into a paste. 
Instead of preparing the bibasic acetate the neutral acetate may 
be used ; in this case, 189 Ibs. are ground with 229 Ibs. of 
litharge and 21 Ibs. of water for a few hours, and then left for 
48 hours. In either case there is formed the tribasic acetate of 
lead. The mass is dissolved in 10 times its weight of water, and, 
then, for every 112 Ibs. of litharge in the mass 84 Ibs. of solid 
bicarbonate of soda is added ; this precipitates the white lead, 
which is finished in the usual way. A modified process was 
described in a later patent. One part of acetic acid of specific 
gravity 1-045 is mixed with 12J times its weight of water, and 
the dilute acid so obtained allowed to act upon granulated lead 
until a solution of specific gravity 1 '040 is obtained ; this is 
mixed with water, and, then, for every 60 Ibs. of acid used in 
preparing the solution, 30 Ibs. of solid bicarbonate of soda are 
added, and the white lead is precipitated. 

The white lead prepared by this process has been favourably 
spoken of; it has a good colour and covering power. In chemical 
composition it resembles white lead, but the process appears to 
be somewhat variable in its results, and, therefore, not com- 
mercially practicable. 

5th GROUP. MISCELLANEOUS PROCESSES. 

Besides the processes described above, others have been 
proposed or patented from time to time which are perhaps 
just worth mentioning, as showing what has been done by 



MISCELLANEOUS PROCESSES. 37 

inventors towards the preparation of white lead by other 
means than the old Dutch process. Some of these processes 
do not come within the groups of processes described above, 
others fall into one or other of them ; but as they are only of 
small importance, and as, in some cases, it is doubtful whether 
they were ever worked on a large scale, they have been relegated 
to this division of white-lead processes for description. The pro- 
cesses are rather numerous, and will only be given in outline; 
for further details the reader is referred to the records of the 
Patent Office. 

Torassa proposed a curious process, which is of interest on 
account of its novelty only, not from any practical value it 
may possess. Lead is granulated, and then placed, with a 
small quantity of water, in a revolving box, or a box fitted with 
agitators ; in this it is worked until it forms a very fine mud, 
which is again agitated with air until it has been converted into 
white lead. The process must have been a slow one, as the 
amount of carbonic acid in the air is small, and can only convert 
in a given time but a small quantity of fine lead into carbonate. 
Wood, a more recent inventor than Torassa, proposed to use 
the same process, but to hasten the preparation of the white 
lead by agitating the fine lead mud with carbonic acid ; but 
even this was not sufficient to make the process a practical 
success. 

Mullins proposed to make white lead by an ingenious but, 
from a practical point of view, unsuccessful method. Sponges, 
saturated with a solution of basic acetate of lead, were suspended 
by porous strings in a chamber into which carbonic acid was 
passed ; this, of course, transformed the basic acetate into basic 
carbonate of lead. The sponges were kept saturated with a 
solution of acetate by connecting the porous strings with a 
tank containing the solution, which, by capillary attraction, 
passed along the strings to the sponges. The process was not 
used on a large scale. 

Martin prepares carbonate of lead so that it shall contain a 
slight excess of carbonic acid. Hydroxide of lead is prepared 
by thoroughly agitating granulated lead with air and water. 
The two bodies are mixed together in the proportion of 8 Ibs. 
of hydroxide to one ton of carbonate, the mixture being made 
by grinding with water into a paste. 

Lewis prepares what he calls white lead from lead or lead 
ores, by mixing these with anthracite coal, and heating the 
mixture in a Wetherill zinc furnace with a powerful blast of 
air; the white lead sublimes, and is collected (see p. 18). 



38 WHITE PIGMEN 7 TS. 

Although the product is spoken of as white lead, it is probably 
the sulphate of lead. 

Button and Dyar treat the basic nitrate of lead with carbonic 
acid gas. 

Brown and Yoimg take the lead nitrate, and pass a current of 
carbonic acid until the liquor becomes saturated with the gas, 
when caustic soda is added in slight excess ; white lead is pre- 
cipitated ; it is allowed to settle, and, after pouring off the 
supernatant liquor, is digested with lime water, and then 
washed and dried. 

Maxwell-Lyte proposes to use spongy lead in the ordinary 
chamber process, with the view of facilitating the action of the 
gases on the lead, and so making the process more rapid. 

Woolrich used a process not unlike that of Torassa ; he provides 
a revolving box, into which he places granulated lead ; by the 
attrition, which occurs during the revolution of the box, the 
lead is gradually converted into a fine powder ; a solution of 
basic acetate of lead is also placed in the box, and this, to 
some extent, by chemical action facilitates the operation. 
Every twelve hours the action is stopped, and the lead mud 
formed is washed out by means of a current of basic lead 
acetate liquor, through which is afterwards passed carbonic acid 
gas to transform the lead into white lead. 

Ozouf uses a solution of the tribasic acetate of lead, places 
this in a closed vessel fitted with agitators, and then sends in a 
current of carbonic acid gas to precipitate the white lead. 

Cookson has a process not unlike that of Cory ; he constructs 
large chambers, into which he throws a solution of basic acetate 
of lead in the form of a spray ; the spraying being done by 
means of a jet of carbonic acid gas. 

Other methods have been proposed, but they are all modifi- 
cations of those which have already been described. 

COMPOSITION AND PROPERTIES OP WHITE LEAD. 

White lead is sold commercially in two forms. One is a 
heavy white powder, having a specific gravity of about 6-47, 
and weighing about 180 Ibs. to the cubic foot; it is stated that 
some processes yield white lead weighing as much as 200 Ibs. 
to the cubic foot. The other form is that of a paste containing 
about 8 per cent, of linseed oil. 

The chemical composition of white lead has already (p. 17) 
been pointed out. It is a basic carbonate of lead formed by 
the union of two molecules of lead carbonate, Pb C O 3 , with one 



COMPOSITION AND PROPERTIES OF WHITE LEAD. 39 

molecule of lead hydroxide, Pb H 2 O 2 ; this is the composi- 
tion of the best make of Dutch white lead, which has all 
the good properties of white lead in the highest degree of 
perfection. 

It is scarcely necessary to point out that as white lead is 
made by many processes it must necessarily vary in composition ; 
indeed the white leads yielded by the same process do not 
always have the same composition, as is evinced by the 
analyses given here and on p. 19; these have been collected 
from a variety of sources. 

ANALYSES OF WHITE LEADS. 

1. 2. 3. 4. 5. 6. 7. 

Lead monoxide, Pb 0, . 86'35 8593 83'77 85'93 84-42 86'5 86'24 

Carbonic acid, C 2 , . 10'44 11-89 15'06 11-89 14'45 11-3 11'68 

Water, Ho 0, . . . 295 2-01 I'Ol 2-01 1'36 2'2 1-61 



99-74 99-83 99 "84 99 -83 100-23 100 '0 99 "53 

from which the composition of the white leads can be calculated 
to be 

Lead carbonate, Pb C 3 , 63 '35 72-15 91-21 71-93 87 '42 6S'36 70-87 
Lead hydroxide, PbH 2 2 , 36-14 27'6S 8 -21 27'88 12-33 31 '64 28'66 
Moisture, . . . 0'25 ... 0'42 0'02 0'48 

No. 1. English make. Made by the Dutch process ; of very good quality. 

No. 2. English make. Made by the Dutch process ; of good quality. 

No. 3. Krenas white. Made by precipitation with carbonic acid gas ; 
this sample is deficient in body although of good colour. 

No. 4. German make. Precipitated by sodium carbonate ; it is only of 
medium quality. 

No. 5. German make. Precipitated by carbonic acid gas ; of good colour, 
but deficient in body. 

No. 6. German make. Made by the Dutch process ; a good white. 

No. 7. German make. Made by precipitation with carbonic acid gas ; 
quality fair. 

The second form in which lead is sold is that of a paste with 
linseed oil. To make this, the dry white lead, above described, 
is first mixed in a mixing mill, with about 8 to 9 per cent, of its 
weight of raw linseed oil ; then it is run through a grinding mill 
several times, to ensure a thorough mixture of the oil and white 
lead. This form is much favoured by painters, as it is more 
readily miscible with oil and turps to make into paint. 

The following are two analyses of ground white lead : 

Lead hydroxide, Pb H 2 2 , . . 65 "96 71 '14 per cent. 

Lead carbonate, Pb C 3 , . . 25-19 20 '45 ,, 

Insoluble, 0'70 

Oil, 8-34 834 



40 WHITE PIGMENTS. 

In making this ground white lead only the best raw linseed oil 
should be used ; boiled oil is not admissible, as there would be 
too much tendency for the lead to become a hard dry mass 
before it could be used. It is customary to keep ground white 
lead under water to prevent it drying up too rapidly. Besides 
its use by painters, this form of white lead is also largely used 
for other purposes, as a cement for gas-piping, &c. 

White lead is soluble in dilute nitric acid, and in acetic acid 
with effervescence, due to the evolution of carbonic acid gas. 
It is also soluble in boiling dilute hydrochloric acid with effer- 
vescence ; on cooling the solution fine transparent needle-shaped 
crystals of lead chloride separate out. Boiling with sulphuric acid 
decomposes the white lead, insoluble lead sulphate being formed. 

Solutions of white lead in acids give white precipitates of lead 
sulphate with sulphuric acid; of lead chloride with hydrochloric 
acid, soluble on boiling in water; and of lead carbonate with 
sodium carbonate. 

Neutral solutions of white lead give a yellow precipitate of 
lead chromate with potassium bichromate, and a black precipitate 
of lead sulphide with sulphuretted hydrogen and solutions of 
sulphides. 

As a pigment white lead possesses all the good qualities 
desired by a painter viz., good colour, body or covering power, 
and permanency. It is distinguished from all other pigments by 
the ease with which it mixes with oil and by forming a paint 
which readily flows from the brush, whereas most pigments, as 
for instance, barytes, tend to work what the painter calls slimy 
or livery, and streaky; white lead does not exhibit this property, 
but flows freely and evenly from the brush. This feature is due 
to the lead hydroxide in the white lead combining with some of 
the oil and forming a lead soap which, dissolving in the rest of 
the oil used in the preparation of the paint, forms a kind of 
varnish ; this varnish takes up the lead carbonate to which is 
due the body or covering power of the pigment. Sometimes 
this chemical combination between the lead hydroxide and the 
oil extends to the lead carbonate and then the white lead loses 
its opacity and becomes more or less transparent or horny ; the 
conditions most favourable to the production of this change, 
which is of rare occurrence, are not properly known. This fact 
of the white lead forming a chemical combination with the oil is 
well known to colour makers, who have endeavoured, by the 
addition of basic bodies, to bring about a similar action in the 
case of other white pigments, such as zinc white and barytes, but 
so far without any great success. 



COMPOSITION AND PROPERTIES OF WHITE LEAD. 41 

When exposed to light and air white lead is fairly permanent 
and will resist exposure to normal conditions for a great length 
of time; on the other hand, when exposed to the fumes of 
sulphuretted hydrogen and other sulphureous gases, white lead 
turns brown or black through the formation of the black sulphide 
or lead. The production of this body is more likely to occur in 
large towns, such as London and Manchester, where large 
quantities of gas are used for lighting and other purposes, which 
usually contains some sulphuretted hydrogen or other sulphur 
compounds. By oxidation this black sulphide can be transformed 
into the white sulphate of lead ; the only agent which can be 
safely used for this purpose in restoring paintings which have 
become discoloured is peroxide of hydrogen, but the action of 
this body is very slow and is much interfered with by the oil 
which is present. 

White lead can be mixed with all pigments except those which, 
like cadmium yellow, ultramarine or king's yellow, contain 
sulphur ; such pigments sooner or later cause the formation of 
the black sulphide and thus bring about the discolouration of 
the pigment or paint. 

White lead is frequently adulterated, the pigment most used 
for this purpose being barytes, because it more nearly approaches 
white lead in specific gravity, and is, on that account, not so 
readily detected ; whereas the use of whiting or gypsum would 
soon be detected on account of the great difference in weight 
between genuine white lead and white lead adulterated with 
them. 

This adulteration of white lead is exceedingly common and is 
well understood by makers and dealers ; in fact it is the custom 
for makers to send out several qualities of commercial white lead 
distinguished as "genuine," "No. 1," "No. 2," and so on; the 
degree of adulteration being regulated by the price which is paid 
for the product. The question whether this is adulteration is a 
matter of opinion ; if by adulteration one means the admixture 
of cheap products with dear products with a view of deceiving 
purchasers of the latter, then the admixture of barytes with 
white lead under the conditions named is not adulteration, for 
the purchaser knows what he is buying, and only pays a fair price 
for such mixed white leads. The author is, however, of opinion 
that this custom of mixing barytes with white lead would be 
much better honoured by the breach than the observance of it. 






42 WHITE PIGMENTS. 



ASSAY AND ANALYSIS OF WHITE LEAD. 

White lead may be assayed for colour and covering power by 
the usual methods (see Chapter X.). 

Dry White Lead. The purity of this pigment is ascertained 
by dissolving some of the lead in pure dilute nitric acid (1 acid, 
2 water) ; strong nitric acid does not dissolve white lead owing 
to the insolubility of the lead nitrate which is formed in the, 
acid ; the ordinary commercial nitric acid contains sulphuric 
.acid, which would lead to the formation of the insoluble 
sulphate of lead, the production of which might lead to the 
condemnation of a pure sample. 

On adding dilute sulphuric acid to the solution, after diluting 
it with water and filtering off the precipitate of lead sulphate 
thus obtained, no further precipitate should be obtained on suc- 
cessively adding ammonia, ammonium sulphide, and ammonium 
oxalate to the filtrate. 

A white precipitate with ammonium sulphide would indicate 
the presence of zinc white, which is a rare thing to find with 
white lead ; a white precipitate with ammonium oxalate would 
indicate the presence of whiting. 

The insoluble residue, if any, will consist most probably of 
baryfces, as other adulterants (for reasons already pointed out) 
.are rarely used ; still any lead sulphate, china clay, gypsum, or 
strontium sulphate which may be used would also be left as an 
insoluble residue on treating white lead with dilute nitric acid. 
To distinguish these bodies, boil the residue in hydrochloric acid 
and place the solution on one side to cool ; if crystals of lead 
chloride separate out and the solution gives a white precipitate 
with barium chloride, then lead sulphate is present. 

The hydrochloric acid solution should be diluted with water 
and sulphuretted hydrogen passed through it ; the black pre- 
cipitate of lead sulphide which may be obtained can be dis- 
regarded ; this is filtered off and the filtrate boiled for some 
time to concentrate it and to drive off" the sulphuretted hygrogen 
it contains. Then ammonia is added, when a white precipitate 
of alumina may be obtained indicating the presence of china 
clay ; this is filtered off, and to the filtrate is added ammonium 
carbonate, which will precipitate any calcium that may have 
been added, in the form of gypsum or whiting. 

A little of the insoluble residue from the hydrochloric acid 
should be held on a piece of platinum wire in the lower part of 
a Bunsen flame when, if it contains barytes, the flame will be 



ANALYSIS OF WHITE LEAD. 43 

coloured green ; if strontium sulphate is present a crimson flame 
will be obtained. This test is not always easy to carry out, but 
with a little care the coloured flames can be obtained, and they 
are good proof of the presence of the pigments named. 

A quantitative analysis of white lead may be made as 
follows : 

Weigh out 2 grammes and dissolve them in a beaker with the 
smallest possible quantity of pure dilute nitric acid, remove the 
insoluble matter by filtering, wash the residue well with warm 
water, adding the first wash waters to the filtrate, then dry the 
residue, place the filter paper and its contents in a weighed 
crucible and burn the paper ; when completely burnt allow the 
crucible to cool in a desiccator and then weigh it. From the 
weight so obtained deduct the weight of the crucible and of the 
filter-paper ash, the difference is the weight of the insoluble 
residue. 

To the filtrate add dilute sulphuric acid and a little alcohol, 
filter off the precipitate of lead sulphate which is obtained, wash 
it dry and burn it in a crucible as before. By multiplying the 
weight of the lead sulphate so obtained by 0'73554 the weight of 
lead oxide in the white lead can be found. 

The carbonic acid can be ascertained \)y treating 2 grammes of 
the white lead with nitric acid in a Schrotter's or other form of 
apparatus for the estimation of carbonic acid. 

The water may be determined by taking the difference between 
the amounts of lead oxide and carbonic acid thus found and 100. 

Hygroscopic water can be ascertained by heating 2 grammes in 
an oven at 110 to 120 C. until no further loss of weight occurs. 

If the white lead be adulterated with barytes, lead sulphate, 
china clay or some of the other insoluble white pigments, these 
will be left behind as an insoluble residue on treatment with 
nitric acid ; their amount is ascertained by filtering off, washing, 
drying, and burning the residue in a weighed crucible in the 
usual way. Soluble adulterants like whiting, strontium carbonate, 
barium carbonate, and magnesite will be dissolved ; if the presence 
of these is suspected, to the filtrate should be added more dilute 
sulphuric acid, which will precipitate the lead and barium ; this 
precipitate can be filtered off, the two can be separated by boil- 
ing with hydrochloric acid, which dissolves the lead sulphate 
but not the barium sulphate. To the filtrate ammonia and 
ammonium oxalate are added ; this precipitates the calcium and 
the strontium, while the magnesite (if present) will remain in 
solution, and can be precipitated by sodium phosphate. It is not 
necessary to describe in detail the methods of separating these 



44: WHITE PIGMENTS. 

adulterants any further ; some notes bearing on this point will 
be found in the descriptions of each individual pigment, while 
reference should be made to works on quantitative chemical 
analysis, such as that of Prof. Sexton, for fuller details. 

By multiplying the weight of the carbonic acid by 5-05 the 
amount of lead oxide with which it is combined can be calculated; 
the two amounts added together give the quantity of lead carbon- 
ate in the white lead. Deducting the amount of lead oxide 
combined with the carbonic oxide from the total present, and 
multiplying this difference by 0-077, gives the amount of water 
combined with it to form lead hydroxide, and the two amounts 
added together gives the amount of the latter body present. 

Paste White Lead can be quantitatively examined as 
follows : Two grammes are treated with strong nitric acid at a 
gentle heat ; this converts all the oil into an insoluble greasy 
matter, of which no account need be taken ; the process then 
becomes identical with that for dry white lead. Should it be 
desired to ascertain the amount of the oil, then 10 grammes 
must be weighed into a filter paper and placed in a Soxhlet or 
other form of fat-extractor and the oil extracted by means of 
petroleum ether ; the ethereal solution is then run into a weighed 
glass, the ether evaporated off and the oil weighed. If a fat- 
extractor is not available it will suffice to agitate the white lead 
with some petroleum ether in a beaker, allowing the pigment 
to settle, pouring off the liquid into a weighed glass, again 
pouring on more ether, and again allowing the white lead 
to settle ; the ether is poured off into the glass, or the mass may 
be filtered. Finally, the ether is evaporated off as before. 

It is sometimes recommended to burn off the oil from the 
white lead in a crucible, but this course is not so satisfactory as 
treating it with petroleum ether, as not only is the oil burnt off, 
but the white lead is decomposed ; whereas in the ether method 
the lead is left in its original form for further examination if 
necessary. 



SULPHATE OF LEAD PIGMENTS. 

Sulphate of lead, Pb S O 4 , forms the basis of a number of white 
pigments which are made on a large scale and sold under a 
variety of names, such as Patent White Lead, Non-poisonous 
White Lead, Sublimed White Lead, &c. These do not consist 
entirely of lead sulphate but contain other bodies, such as zinc 
oxide, barytes, magnesia, &c., in varying quantities ; they are 



SULPHATE OP LEAD PIGMENTS. 45 

made by different methods, and most of those now sold are pro- 
duced by patented processes. 

Lead sulphate can be made by dissolving lead in strong sul- 
phuric acid, the action is, however, but slight and does not form 
a commercial method of manufacture. It is mostly made by 
adding sulphuric acid to solutions of either lead acetate or lead 
nitrate ; perhaps the best method is that where lead acetate is 
used. 

Metallic lead is granulated by melting and pouring the molten 
lead into cold water ; the object of granulating is to obtain the 
metal in such a form as to expose a large surface to the action of 
acids and air. The granulated lead is placed in a large tub fitted 
with a closed steam coil so that the action of the acid may be 
facilitated by heat, if necessary. Acetic acid diluted with its 
own volume of water is poured on to the lead ; the action of the 
acid is at first rather sluggish, but by allowing the action to go 
on for about 12 hours, then running off the acid and leaving the 
lead in the tub without any liquor, a certain amount of oxidation 
goes on, resulting in the formation of a deposit of oxide on the 
lead, so that when the acid is again admitted to the lead, the acid 
acts more rapidly, and a strong solution of lead acetate is soon 
obtained ; this action is facilitated by a gentle heating of the 
contents of the tub. From the tub the solution is run into a 
large wooden vessel and to it is added strong sulphuric acid in 
small quantities at a time with constant stirring ; lead sulphate 
is thereby precipitated. Care is taken that the amount of sul- 
phuric acid used is not sufficient to throw down all the lead, but 
that some of the latter is left in solution. The lead sulphate is 
allowed to settle, and then the clear supernatant liquor is pumped 
back again into the lead tub, for it contains all the acetic acid ; 
for, as will be seen from the equation 

Pb2C 2 H 3 2 + H 2 S0 4 = PbS0 4 + 2HC 2 H 3 2 
Lead acetate. Sulphuric acid. Lead sulphate. Acetic acid. 

acetic acid is reproduced as the result of the reaction which goes 
on between the lead acetate and the sulphuric acid, and this 
acetic acid can be used over again for preparing fresh solution of 
lead for precipitation ; thus a comparatively small quantity of 
acetic acid may be used to prepare a large quantity of lead sul- 
phate ; theoretically speaking, beyond the first charge no more 
acetic acid is required, but practically, there is a small loss which 
requires to be restored by new additions of acid from time 
to time. 



46 WHITE PIGMENTS. 

The sulphate of lead which is precipitated is washed with 
water, drained on a filter, and dried ; after which it is ready for 
use as a pigment, or can be combined with other white pigments 
if desired. 

The acetic acid used should be as pure as possible ; the usual 
commercial variety of a strength of about 1-050 = 10 to 12 
Twaddell is sufficiently good for the purpose ; this contains about 
35 per cent, of actual acetic acid, although some makes contain 
more. 100 Ibs. of it will dissolve about 60 Ibs. of lead supposing 
all the acetic acid exerts its solvent power, but in practice this 
never or but rarely happens, nor indeed is it necessary that it 
should. From 20 to 28 Ibs. of sulphuric acid will be required to 
precipitate the lead dissolved by this quantity of acetic acid. 
These figures are only approximate and are given simply as 
guides to actual practice. 

Lead sulphate has the formula Pb S O 4 , and contains 

Lead oxide, Pb 0, . . . 73 '55 per cent. 
Sulphuric anhydride, S 3 , . 26 '45 ,, 

and of metallic lead 68-31 per cent. 

It is a white, somewhat crystalline, and very heavy powder, 
its specific gravity being about 6 -3. It is only slightly soluble 
in water, insoluble in dilute acids and in alcohol, but soluble in 
solutions of ammoniacal salts, and in strong sulphuric acid j from 
the latter solution it is precipitated on the addition of water. 
Boiling concentrated hydrochloric acid dissolves it, and crystals 
of lead chloride fall down as the solution cools. 

As a pigment it is not satisfactory, its crystalline character 
reduces its body and covering power, causing it often to work 
streaky or livery under the brush ; this defect can be remedied 
to some extent by grinding it. It is not readily acted upon by 
sulphuretted hydrogen, and is, therefore, more permanent than 
white lead under exposure to air. Owing to its solubility being 
less it is free from the poisonous character of white lead, and, 
therefore, white pigments containing it are often sold as " non- 
poisonous white leads." Its colour or hue is a good white, but 
slightly yellower in tone than white lead and about equal to- 
"barytes. 

It is used as a diluent in the manufacture of pale chromes. 

Many have been the attempts to make lead sulphate the base 
of commercial white leads, the records of these are to be found 
in the publications of the Patent Office, where they lie buried in 
an almost unknown condition, and it would really be most 
instructive for colour-makers and would-be inventors if they 



SUBLIMED WHITE LEAD. 47 

would peruse these records and see what has been done in the 
past. A few of these inventions may be briefly noticed here, 
and a fuller description given of such as are at present used on 
a large scale. 

Richardson, in 1839, patented* the use of the sulphate only 
as a pigment; in 1853, Carter & Marriott prepared a chloro- 
sulphate of lead made by treating 100 Ibs. of litharge witli 
25 Ibs. of salt, and the product so obtained with 5 Ibs. of sulphuric 
acid. Woods, in 1866, took out a patent for the preparation of 
a white pigment from lead fume, which is a mixture of lead, lead 
oxide, and lead sulphate ; this he treated with hydrochloric acid 
thus forming a chloro-sulphate j or he calcined the fume in a 
furnace, whereby it was converted into a white mixture of lead 
oxide and lead sulphate, which was then treated with hydro- 
chloric acid as before. Groves, in 1826, treated galena 'with 
potassium nitrate and sulphuric acid, whereby the lead sulphide 
was converted into lead sulphate, which was dried and sold as a 
pigment. In 1866, Messrs. Bell & Fell patented the use and 
preparation of what they called a sub-sulphate of lead, prepared 
by precipitating a solution of lead nitrate with sulphuric acid 
and then boiling with an alkali. 



SUBLIMED WHITE LEAD. 

This product is the invention of G. T. Lewis, and was first 
patented in 1879. About 1870 a vein of ore containing both 
lead and zinc, being a mixture of galena and blende, was 
discovered in America ; this was smelted for lead, which, owing 
to it being different in properties from ordinary lead, was dis- 
tinguished as " Bartlett lead ; " the presence of zinc in lead ores is 
very detrimental, as the zinc cannot wholly be removed from the 
lead, while it imparts to it properties which, by causing it to- 
become hard and brittle, prevent its application to those uses for 
which lead is of great service. The process of white-lead making" 
now to be described was to some extent devised to utilise this- 
lead-zinc ore. 

The manufacture of sublimed lead depends upon two facts,, 
which also are the principles that underlie some of the ordinary 
processes of lead smelting first, when lead ore (galena, lead 
sulphide) is heated in a furnace with access of air it. undergoes 
oxidation, partly to lead oxide, Pb O, partly to lead sulphate, 
Pb S O 4 , the amount of the oxidation depending upon the amount 
of air which comes in contact with the ore ; if this is small then 



48 WHITE PIGMENTS. 

the sulphate is mostly formed. The temperature also has some 
influence on the result ; in fact, it is probably the chief factor in 
the process. If low, then the sulphate is chiefly formed ; if high, 
this is likely to be decomposed into oxide and sulphur dioxide. 
Secondly, during the operation of lead smelting a large pro- 
portion of the lead is carried off in the form of " fume," which 
collects in large chambers or flues built for the purpose. This 
lead-fume is a mixture of metallic lead, lead oxide, lead sulphate; 
if there be any zinc in the ore it will be mostly found in this 
fume. It has frequently been noticed that the composition of 
the fume varies from time to time, according to the conditions 
under which the furnaces are being worked. Lewis proposes to 
utilise the two principles laid down and, by carrying out the 
process in such a manner that the great bulk of the fume 
produced consists of lead sulphate, to prepare a white pigment 
from any lead ore. This is effected in the Lewis and other 
processes based on the same principle by causing the operation 
to be carried out in a blast furnace. 

The preparation of sublimed white lead takes place in two 
stages ; the first may be called the fume stage, in which the lead 
ore is transformed into fume of more or less complex com- 
position ; the second stage may be called the colouring stage, 
in which the pigment is made of a proper degree of white- 
ness. 

In the first stage a furnace is used resembling the Wetherill 
zinc furnace. Four of these are placed back to back and side 
by side, and is hence not unlike four American lead-smelting 
hearths. The furnace is worked from two sides, known as 
the fronts; a water back, containing tuyeres, is also placed 
between the bench of furnaces; above the water back is an air 
chamber divided into two compartments by a partition, so 
arranged that the air which enters into one of these compart- 
ments passes to one front of the bench of furnaces, while the 
air in the other passes to the other front. This air is heated 
by passing through the hot walls of the furnaces, and by 
suitable means is sent as a powerful blast through the tuyeres 
into the hearth of the furnace, and through any ore which 
may be on the hearth. The ore is placed on the hearth of 
the furnace with the fuel necessary to effect its melting, and 
air is sent through it; part of the lead is reduced to the 
metallic state, and is collected by being allowed to flow out of 
the furnace into a suitable receptacle. Some of the lead is 
blown by the blast of air into fume, and is more or less 
oxidised to oxide and sulphate during that operation; this fume 



SUBLIMED WHITE LEAD. 49 

passes up into the collecting hood which is placed above the 
hearth, and from thence into a large chamber, where it collects 
on the sides. These chambers are built in two storeys of brick 
and iron, the openings connecting the two storeys are covered 
by means of woollen bags ; a powerful fan in the second storey 
draws the gases produced during the process of furnacing 
through the bags, but the latter retain the lead fume, which 
therefore collects in the lower storey of the chamber. The 
woollen bags are shaken from time to time to dislodge 
the fume which collects on them. At intervals the fume 
in the lower storey is set fire to, a proceeding which com- 
pletes the oxidation of the lead fume, and, at the same time, 
makes the fume more consolidated, and, therefore, better for use 
as a pigment. The burnt fume has a lead colour, and requires 
further treatment before it can be used as a white pigment, 
although it is sold in this state as a lead-grey colour. 

The next operation, in which the pigment is made white, 
takes place in the "slag-eye furnace" as it is called. This slag 
furnace consists of a square brick chimney, fitted with a hearth, 
tuyeres, and water back. In this furnace the fume from the 
last operation is heated in a powerful blast of hot air, which 
causes most of it to become completely oxidised to lead sulphate, 
which passes into condensing chambers, where it collects. These 
chambers are constructed in exactly the same way as those 
described above. As a rule, the pigment is ready for sale, but 
occasionally it may have a slight grey colour; to get rid of 
this it is treated with sulphuric acid, which causes it to whiten, 
and sometimes it is ground in order to render it as fine as 
possible. 

Sublimed white lead is a powder of a fine white colour, 
although sometimes it has a grey tint ; the white has a rather 
bluish hue. It is heavy, its specific gravity being over 6 ; 
a cubic foot weighs about 200 Ibs. It is quite insoluble in 
water, is partially soluble in dilute nitric acid, which dissolves 
out the zinc oxide, lead oxide, or lead carbonate the pigment 
may contain ; as a rule, it is completely soluble in boiling 
hydrochloric acid, although some samples contain a little 
barytes, which remains behind as an insoluble residue on treat- 
ment with the acid. 

In composition it is very variable. Much depends upon the 
ore from which it is made, and whether anything has been added 
to it after the pigment was made in the furnaces described above. 
The following analyses are partly from published accounts, and 
partly from the author's own results : 



50 WHITE PIGMENTS. 

I. II. 

Lead sulphate, Pb S 4 70 82-390 per cent. 



Lead oxide, Pb 0, 
Zinc oxide, Zn 0, . 
Lead carbonate, Pb C 3 , 
Water, hygroscopic, 
Water, combined, 



23 0-554 
7 6-335 
9-421 
0-350 
1-050 



The proportion of oxide of zinc and sulphate of lead contained 
in the pigment is, to some extent, a measure of the value of the 
pigment for painting purposes. 

Sublimed white lead when well made is a fairly satisfactory 
pigment. It possesses good colouring power and body or covering 
power, although in this respect it is perhaps not quite equal to 
the best white lead. It is more permanent than white lead, 
because sulphureous gases and vapours have little or no action 
upon it ; it, therefore, keeps its colour longer when used in paint 
and exposed to air. Being insoluble in water and acids it is not 
so poisonous as white lead. 

It has been noticed that occasionally when mixed with oil and 
turps, sublimed white lead has a tendency to become gelatinous ; 
on what this peculiar property depends is somewhat uncertain. 

FREEMAN'S NON-POISONOUS WHITE LEAD. 

The base of this white lead is the sulphate made from metallic 
lead by precipitation, it also contains zinc oxide, barytes, and, in 
the earlier makes, a little magnesia, all the ingredients being 
mixed together by a process of grinding under edge runners, 
which causes them to be thoroughly incorporated together, and, 
at the same time, by consolidating the materials, increases the 
body or covering power. The pigment was first patented in 1882, 
but since then other patents have been taken out for improve- 
ments in its composition, which, in consequence, has varied a 
little from time to time. The non-poisonous white lead is a very 
good pigment, is more permanent under exposure to atmospheric 
influences than white lead, and is equal to white lead in body or 
covering power and in freedom of working. It is rather heavier 
than white lead, weighing about 180 to 190 Ibs. to the cubic 
foot. Its specific gravity is 5 '9 5 to 6-00. It is one of the best 
substitutes for white lead which have been made. 

HANNAY'S CALEDONIAN WHITE LEAD. 

Messrs. Andrew French & J. B. Hannay in 1884 took out a 
patent for the preparation of a chloro-sulphite of lead to be used 



HANNAY'S CALEDONIAN WHITE LEAD. 5l 

as a substitute for white lead. This was an extension of a 
former process of French's for the production of a sulphite of 
lead pigment (see p. 53). 

To prepare the chloro-sulphite of lead, lead ore, lead-fume, or 
any lead compound is mixed with coke, and, if the amount of 
sulphur in the ore or lead used is deficient, with some pyrites, as 
it is necessary that a large quantity of sulphurous acid (sulphur 
dioxide) gas should be produced. This mixture is placed in a 
kind of cupola furnace provided with an air blast. The heating 
of the furnace and the amount of air blown through is to be so 
arranged that the lead is oxidised to sulphite, not to sulphate ; 
the sulphite is carried by the blast into a chamber and along flues 
where it comes into contact with hydrochloric acid gas, which, by 
its action on it, produces the chloro-sulphite. The hydrochloric 
acid gas is formed by sending a solution of salt into the flues in 
the form of a fine spray ; the sulphurous acid in the furnace gases 
acting on the salt under such conditions decomposes it into 
hydrochloric acid and sulphite of sodium. From the flues the 
pigment is carried into large condensing chambers where it col- 
lects ; from time to time the chambers are emptied of their con- 
tents, and the pigment washed with water and dried, when it is 
ready for use. 

In a subsequent patent Mr. Hannay describes the production 
of the sulphate of lead from lead-fume, &c., by heating in a cupola 
furnace with a blast of air. The fume which is thus produced is 
carried through a kind of reverberatory chamber, where it is 
completely oxidised to sulphate ; this is carried forward by the 
draught into large chambers where it collects. If white enough 
the pigment so obtained is sent out for use ; if it has a grey 
colour due to the presence of metallic lead it is bleached by 
treatment with an acid. To improve the quality of the pigment 
it may be mixed with zinc oxide. 

One difficulty which is met with in all these subliming 
processes is that of collecting the lead-fume or pigment. This 
necessitates the use of large condensation chambers or long flues 
to ensure the complete deposition of the material, or otherwise 
there is great risk of much of the material being carried forward 
by the draught into the chimney of the works and so out into the 
atmosphere where it is lost. 

To provide these large chambers or long flues is not always 
possible, both on account of the cost of construction and of the 
space they require. To remedy this, Mr. Hannay has devised a 
special form of condenser constructed as follows : A large closed 
chamber made of iron is partially filled with water, the flue from 



52 WHITE PIGMENTS. 

the furnace passes through the top of the chamber and terminates 
a little below the surface of the water, so that all the gases from 
the furnace must pass through the water. On each side of the 
flue in the water is arranged a number of metallic gauze screens 
in such a way that the gases, &c., must pass through them on 
their way from the flue. These screens serve to collect the fume 
and solid matter which comes from the furnace ; from time to 
time the screens are shaken by the mechanical contrivance 
provided for that purpose, which frees them from the fume that 
has deposited upon them and this settles down to the bottom of 
the tank or water chamber. The flue gases which pass through 
the screens are drawn through them and the water into a 
chimney by means of a fan. From time to time the fume that 
deposits is collected and treated as may be required to convert it 
into a useful pigment. 

The properties of the pigment thus made do not differ much 
from those of the sublimed white lead described above. 



MAXWELL LYTE WHITE LEAD. 

In 1886, Mr. Maxwell Lyte patented the production of a basic 
sulphate of lead for use as a pigment. The process of manu- 
facture is given as follows: A solution of basic acetate of lead 
is made by pouring three-fourths of a solution of the normal 
acetate of lead of a specific gravity of 1*15, 30 Tw., over 
spongy lead, by which means a solution of basic acetate, having 
a specific gravity of 1*315, or 63 Tw., is obtained. 100 gallons 
of this solution are taken, and to it is added just enough sulphuric 
acid to turn litmus paper red ; the mass is then boiled, the 
sulphate allowed to settle, and the clear liquor used for making 
new solutions of lead. The precipitate of lead sulphate obtained 
is then treated with a solution of basic acetate of lead, being 
boiled therewith for an hour, when the basic sulphate will be 
formed. 

The sulphate of lead pigments may be distinguished by boiling 
them with hydrochloric acid, which will, if free from barytes, 
dissolve them completely ; any barytes they may contain will be 
left as an insoluble residue. The solution will yield a white 
precipitate of barium sulphate on adding a solution of barium 
chloride, and will deposit crystals of lead chloride on cooling. 
The presence of lead carbonate, zinc oxide, or other white pig- 
ments, may be detected in either a nitric acid or hydrochloric 
acid solution by the tests given under white lead. 

A full analysis of these lead sulphate pigments is rarely re- 



ZINC WHITES. 53 

quired ; but they should be assayed for colour, hue, covering 
power, &c., by the methods described in another chapter. 

SULPHITE OP LEAD WHITE. 

Sulphite of lead is a white insoluble powder containing lead 
oxide and sulphur dioxide, and having the formula PbSO 3 . 
It was first proposed to be used as a pigment in 1850 by Dr. 
John Scoffern, who took out a patent for its production and use 
for painting. The process he describes for its preparation is to 
prepare a solution of basic acetate of lead, and to precipitate the 
sulphite from this by passing a current of sulphur dioxide gas 
through it. A solution of normal acetate is left, which may be 
used for preparing fresh basic acetate ; while the pigment which 
is precipitated is collected, washed with water, and dried. This 
sulphite white is said to be of good colour and body, to mix 
better with oil than the sulphate, and to keep its colour better 
than the carbonate white. 

In 1881 Andrew French patented a process of manufacturing 
a white pigment, which consisted of a mixture of about 80 per 
cent, of sulphite of lead, with 20 per cent, of oxide of zinc, al- 
though these proportions were liable to vary from time to time. 
This pigment was made by mixing ores containing both lead 
and zinc with carbon in such forms as coal, charcoal, coke, <fec., 
and treating the mixture in a low cupola furnace fitted with 
a hot blast. By this treatment the ore becomes partly reduced 
to the metallic state ; while some is oxidised and carried by the 
blast of air in the form of fume into flues and collecting chambers, 
where it settles out. The amount of air supplied in the blast 
is so arranged that the oxidation is limited, and the sulphites 
of the metals are produced and not the sulphates (see Sublimed 
and Hannay's white leads). The pigment so made is described 
as being superior to ordinary white lead, inasmuch as it retains 
its colour longer and is superior to sulphate whites in having 
more covering power or body, and mixing with oil better. 

The author has not seen any of these sulphite whites, and they 
are not now made. 

ZINC WHITES. 

Several white pigments containing zinc as their base are now 
used as pigments on a fairly large scale. These usually contain 
as their essential constituent one or other or both of two com- 
pounds of zinc viz., the oxide, Zn 0, and the sulphide, Zn S ; 



54 WHITE PIGMENTS. 

the former has been used as a pigment for a long time, the latter 
is of comparatively recent introduction. The oxide is generally 
sold under the name of zinc white, and is always the body or 
compound which is understood to be designated by that term. 
The sulphide is rarely sold pure, but is more or less mixed with 
oxide of zinc and other white pigments, and sold under a variety 
of names, which will be given below. 



ZINC WHITE. 

This pigment, also known as Chinese white, consists entirely 
of the oxide of the metal zinc, which has the composition 

Zinc, Zn, . . . 8O25 per cent. 
Oxygen, 0, . . . 19 '75 

having the formula Zn 0. 

It can be prepared in two ways 

1. By Combustion of Metallic Zinc. Zinc is a volatile metal, 
and when heated to a white heat it is readily converted into 
vapour ' } if this comes into contact with air or oxygen combustion 
can take place, and the oxide is formed by the union of the metal 
with oxygen, as shown in the equation 

Zn + O Zn 0. 

The usual method of manufacturing zinc white is based on 
this reaction. 

2. By the Action of Heat on certain Zinc Compounds. When 
the carbonate or hydroxide of zinc are heated strongly they lose 
part of their constituents, leaving a residue of oxide behind. 
This method of preparing zinc white has been used and made 
the subject of many patents. 

1. Combustion Method. This is the principal method for 
preparing zinc white. The process is carried out in a special 
form of plant, which is shown in Figs. 8 and 9. This plant 
consists essentially of two portions, one in which the zinc is 
produced in the form of vapour, and the other of chambers in 
which the white formed by the combustion of the zinc vapour 
is collected. The vapourising plant consists of a furnace, A A, 
in which are placed a number of fireclay retorts, B B B. These 
furnaces are made double, back to back, so as to accommodate two 
sets of retorts, as shown in the drawing ; they are of the rever- 
beratory type. The retorts are made of fireclay ; the usual form 
is shown in the drawings, although it varies a little at different 
works. The mouths of these retorts open into a kind of com- 



MANUFACTURE OF ZINC WHITE. 



55 



bustion chamber or flue made of sheet-iron; one end of this 
flue is made funnel-shaped, and is open to the air, so that a 
current of air can pass through it, the other end of the flue opens 
into the first collecting chamber. The white hot vapour of zinc, 




Fig. 8. Zinc white furnaces. 

when it comes from the retort into this flue, burns, and vapour 
of zinc oxide is emitted in large volumes ; this is carried by the 
draught into the collecting chambers. The collecting chambers, 
C C C, are large chambers varying in number at different works, 



c' 



c' 



c" 




Fig. 9. Zinc white furnaces. 

but usually six set in two series are employed. They are con- 
structed of wood, and each chamber is complete in itself; it com- 
municates with its neighbours on either side by means of small 



56 WHITE PIGMENTS. 

apertures so placed alternately at top and bottom that the vapours 
and gases from the retorts must take a circuitous course through 
the series of chambers; in so doing the oxide of zinc which they 
carry with them is deposited on the sides of the chambers ; finally, 
the gases pass through a long flue, G G G, to the chimney of the 
works. In this flue there are screens which serve to intercept 
any oxide which has escaped condensing in the chambers. The 
bottom of the chambers is made hopper-shaped, so that, by placing 
barrels underneath and opening a slide, the oxide can be taken 
out of the chambers with ease. Although zinc-white chambers 
are usually constructed of the form shown in Figs. 8 and 9, 
yet it is obvious that any other form of collecting chambers may 
be used, such as, for instance, those described under lamp-black. 
The retorts measure about 28 inches long by 10 inches broad, 
and 6 to 8 inches high, the sides having a thickness of 1 1 inches. 
Eight of these retorts are placed in a furnace arranged in two 
sets of four, back to back ; two retorts open into one series of 
three collecting chambers. 

The furnace is so constructed that the flames from the fireplace 
first pass over the retorts, then under them in a flue, and finally 
pass out, or rather the products of combustion pass out, into a 
chimney by the flue, G G G, connected with the collecting 
chambers, whereby the draught necessary for drawing the zinc 
oxide through the chambers is created. The draught should be 
good, so as to ensure a plentiful supply of air to the vapour of 
the zinc as it passes out of the retorts, otherwise some of the 
zinc is sure to escape combustion, and this, passing into the 
chambers unchanged, causes the discolouration of the white. 

Usually metallic zinc is used as the raw material for the pre- 
paration of the zinc white, but it has been proposed to use zinc 
ores, such as calamine (carbonate of zinc), and zinc blende, or black 
Jack (sulphide of zinc), and any products containing zinc, which, 
by being heated in a retort, can be reduced to the metal, which, 
being volatilised in the operation, can be burnt and collected in 
the manner described above. The great objection to this method 
of procedure is that the zinc ores are rarely pure, being accom- 
panied by varying quantities of other metals cadmium, antimony, 
<fec. Some of these are volatile like zinc, and on burning form 
oxides which pass into the collecting chambers and contaminate 
the zinc white, spoiling the colour very considerably. This is 
especially the case with cadmium, which is a very common con- 
stituent of zinc ores. This metal forms a brown oxide, which, 
if present, always spoils the zinc white. In the ordinary process 
of zinc smelting it is eliminated by taking advantage of its greater 



MANUFACTURE OF ZINC WHITE. 57 

volatility, but it is obvious that in zinc-white making it is not 
possible to separate it out from the finished product. It is chiefly 
on account of the presence of these impurities in the zinc ores, 
and the discolouring action they have on the white, that they 
are not now used for preparing it. 

Occasionally it is found that the white collected in the first 
chambers is not so good as that collected in the following ones ; 
this is due to the presence of small traces of impurities in the 
zinc which is used and which it is difficult to entirely eliminate. 
These impurities are found to collect mostly in the first chamber, 
and the white in this is, when found to be of a poor colour, sold 
as an inferior quality of zinc white. 

The general methods of working is to raise the retorts to a 
white heat, and then to throw in the ingots of zinc ; all 
apertures are now closed up excepting those leading from the 
air to the combustion chamber and into the collecting chambers. 
The zinc soon begins to volatilise, and on issuing from the mouth 
of the retort burns. It is essential to prevent the combustion 
taking place within the retort because it would then soon 
become choked up by a deposit of infusible oxide. This can 
only be done by making the mouth of the retort narrow, and 
scraping the crust formed on the mouth of the retort by means 
of scrapers. The zinc white formed by the combustion of the 
vapours is of two kinds, light and heavy ; the former passes into 
the chambers and is there collected, the latter drops down the 
combustion chamber, being rather heavy, into a barrel placed 
for its reception, and being usually of a poor colour, may be 
returned to the retorts with the next batch, along with a small 
quantity of carbon in some form or other. 

The zinc white made by this process, while being of a good 
colour, is very light and, in consequence, is rather deficient in 
body, still it gives a more uniform product than any other 
process hitherto devised, and is, therefore, the chief, if not the 
only, process now in use for making zinc white. 

2. Ignition Methods. When carbonate of zinc, Zn C O 3 , is 
heated it loses its carbonic acid and leaves a residue of zinc 
oxide, Zn 0, according to the equation 

ZnC0 3 = ZnO + C0 2 . 

Similarly the hydroxide, ZnH 2 O 2 , loses water and leaves, 
oxide behind 

ZnH 2 2 




UNIVERSITY 



58 WHITE PIGMENTS. 

Other salts of zinc, such as the nitrate (Zn 2 N O 3 ), the sulphate 
(Zn S O 4 ), the sulphide (Zn S), are also altered when heated and 
yield a residue of oxide ; but they require rather a higher 
temperature to effect their decomposition than do the first two 
compounds named. 

Neither of these two compounds occurs naturally in a suffi- 
ciently pure form to be used as a source for the preparation of 
zinc white by an ignition method, therefore they have to be 
prepared artificially ; the carbonate, by precipitating, by means 
of a solution of sodium carbonate, any solution of zinc, when 
it is obtained as a white powder insoluble in water. The 
hydroxide is similarly obtained by adding a solution of caustic 
soda to a solution of zinc. In this case care has to be taken not 
to add the precipitant in excess or otherwise some of the 
hydroxide first thrown down will be redissolved again, zinc 
hydroxide being soluble in caustic soda solution. The pre- 
cipitates in either case are well washed with water by decan- 
tation, and then drained on a filter. It is not necessary to dry 
them. When as much water has drained from them as possible, 
the mass is thrown into a crucible and heated to a white heat 
for some time, when a bright yellow residue of zinc oxide is 
left in the hot crucible ; this, however, as it cools turns white. 
When cold the oxide is ground up for use as a pigment. The 
white obtained by this process is a good one, has good body or 
covering power and is of good colour; but the process of 
manufacture is more costly than that by combustion, con- 
sequently, it is rarely, if ever, used. Attempts have been 
made to utilise by this method many waste solutions of zinc 
which are obtained in various industries, but hitherto without 
much success. The principal difficulties are the trouble of 
collecting these liquors and the influence which any impurities 
they contain may have on the colour of the resulting pigment, 
these impurities being difficult, if not impossible, to eliminate 
by any practicable process. 

PROPERTIES OP ZINC WHITE. 

Zinc white is a rather bulky fine white powder, having a 
specific gravity of 5-6. In hue it is rather bluish. It is 
quite insoluble in water, oil, alcohol, and turpentine. It dis- 
solves in dilute sulphuric acid, hydrochloric acid, acetic acid, 
and many other acids without effervescence, giving colourless 
solutions. It is also soluble in ammonia and alkaline solutions. 
It is unchanged by exposure to air and light, and sulphur 



ASSAY AND ANALYSIS OF ZINC WHITE. 59 

or sulphur gases have no visible action on it, as the sulphide 
formed is white like the oxide ; as a pigment, therefore, it is 
quite permanent. It mixes well with all vehicles used in pre- 
paring paints. For preparing the stiff paste, which is sometimes 
sold, it requires about 22 per cent, of linseed oil, a much larger 
proportion than is required by any other pigment. It mixes 
well with water, arid in this form it is largely used by water- 
colour artists, under the name of Chinese white. It can be 
mixed with all other pigments without undergoing any change 
or changing the other pigment. The chief fault of zinc white is 
its want of body or covering power. This is, no doubt, due to its 
bulkiness, which is inseparable from the mode of preparation. 
Many attempts have been made to produce a dense zinc white. 
The plan most commonly in use is to grind the pigment for 
some time under edge runners, so as to break down, so to 
speak, the voluminous character of the pigment, and make it 
more powdery, in which form it has greater covering power. 
Another proposed plan is to heat the zinc white to a red heat, 
and then drop it into cold water ; the sudden cooling effects the 
breaking up of the particles of zinc white, but this method is 
not so effective as the grinding process, and is rather more 
troublesome to carry out. This want of body, coupled with 
the fact that it is rather costly, has limited the use of zinc 
white much below that justified by its permanent qualities. 

ASSAY AND ANALYSIS OP ZINC WHITE. 

Zinc white can be assayed for covering power and colour in 
the usual way. 

Zinc white, being a rather costly pigment, is very liable to 
adulteration with other white pigments, such as china clay, 
barytes, whiting, terra alba, &c. 

Zinc white should be completely soluble in dilute sulphuric 
acid without effervescence ; a property possessed by no other 
white pigment. Effervescence indicates an addition of white 
lead, whiting, or magnesite. Most other adulterants are left as 
an insoluble residue. The solution in acids is quite colourless. 
On adding ammonia, a white precipitate is first obtained; but, 
on adding an excess, this is redissolved; any lead, if present, 
is left as an insoluble residue. On adding ammonium sulphide 
to the ammoniacal solution, a white precipitate of zinc sulphide 
will be obtained ; this precipitate should be quite white, or, at 
most, have a faint yellow tint, due to its containing traces of the 
yellow ammonium sulphide ; any other colour would indicate 



60 WHITE PIGMENTS. 

impurities. This precipitate should be filtered off, and to the 
filtrate a solution of ammonium oxalate be added ; no preci- 
pitate should be obtained; if there is any it indicates the 
addition either of calcium carbonate in the form of whiting 
(which is also indicated by effervescence with acids), or of 
calcium sulphate. If to the solution or filtrate from the 
ammonium oxalate sodium phosphate be added, the forma- 
tion of a white precipitate indicates the addition of magnesium. 

If a preliminary test with dilute sulphuric acid shows 
that adulterants are present, it will be best to treat the 
pigment with hydrochloric acid, and boil it, filtering off any 
insoluble residue, and then allowing the solution to cool ; if 
crystals form, then lead is present, and has been added either 
in the form of white lead or lead sulphate, the character of the 
results obtained will indicate which. In the solution zinc, 
calcium, and magnesium may be looked for as described above. 
If an insoluble residue is left it may contain barytes, china clay, 
or gypsum ; which substances may be tested for in the way 
described under their respective heads. 

The quantity of zinc oxide in a pigment can be thus ascer- 
tained Weigh out about 1 gramme of the pigment, dissolve it 
in hydrochloric acid, and to the solution add first ammonia in 
excess, then ammonium sulphide in slight excess ; the mixture 
is placed on one side for a few hours, and then filtered ; the pre- 
cipitate of zinc sulphide is well washed, dried, placed in a por- 
celain crucible, and ignited at a bright red heat, taking care that 
plenty of air reaches the material in the cruicible ; by this means 
the sulphide is converted into oxide. When the reaction is con- 
sidered to be complete, the crucible and its contents are allowed 
to cool, and, when cold, are weighed ; the weight of zinc oxide 
found gives at once the weight of zinc white in the pigment. 

SULPHIDE OP ZINC WHITES. 

When ammonium sulphide is added to alkaline solutions of 
zinc salts, or when sulphuretted hydrogen is passed through 
similar solutions, a white precipitate of the sulphide of zinc is 
obtained. This is a body composed of sulphur and zinc in the 
following proportions 

Zinc, .... 67 per cent. 
Sulphur, . . . 33 

its formula being Zn S. 

The first mention of the use of this compound as a pigment 



ORE'S WHITE ENAMEL OR CHARLTON WHITE. 61 

occurs in a patent granted in 1852, the process for its preparation 
being to add a solution of sulphide of potash to one of zinc sul- 
phate. The patentee did not prepare it with a view of using it as 
a painters' pigment, but for the purpose of colouring india-rubber. 

During the last twenty years very considerable attention has 
been paid to zinc sulphide as the possible base of a white pigment. 
The consequence has been that it is now manufactured on a large 
scale by several, colour manufacturers and sold as a pigment 
under a variety of names, as Orr's Charlton white, Orr's enamel 
white, Griffith's patent zinc white, patent zinc white, lithophone, 
&c. None of these bodies consist only of sulphide of zinc, but 
contain oxide of zinc, barytes, magnesia, &c. 

When alkaline sulphides are added to solutions of zinc salts, a 
white precipitate of the sulphide is obtained, according to the 
equations 



ZnS0 4 + Na 2 S = Zn S + Na 2 S 4 

Zinc Sodium Zinc Sodium 

sulphate. sulphide. sulphide. sulphate. 

ZnCl 2 + CaS = Zn S + Ca C1 2 

Zinc Calcium Zinc Calcium 

chloride. sulphide. sulphide. chloride. 

which may be considered as typical of the operations carried out 
on a large scale. 

Zinc sulphite possesses what to the painter is of great impor- 
tance, "body;" the colour is apt not to be good. It is extremely 
difficult to remedy this defect with pure zinc sulphide, but by 
introducing other bodies and preparing the sulphide in a special 
way, a pigment possessing both good colour and good body may 
be obtained. 



ORB'S WHITE ENAMEL OR CHARLTON WHITE. 

This zinc pigment was first patented in 1874, and is the parent 
of this group of zinc whites. The process of manufacture consists 
in first preparing barium sulphide by calcining barytes with 
charcoal for some hours at a white heat ; the calcined mass is 
then lixiviated with water to dissolve out the barium sulphide 
which is formed. The solution so obtained is divided into two 
equal portions. To one of these is added a solution of zinc 
chloride, whereby a precipitate of zinc sulphide is obtained 
according to the equation 

BaS + ZnCl 2 = BaCl 2 + Zn S. 
Barium Zinc Barium Zinc 

sulphide. chlond*. chloride. sulphide. 



62 WHITE PIGMENTS. 

The precipitate is not separated, but to the mass is now added 
the rest of the barium sulphide and sufficient solution of zinc 
sulphate, when a combined precipitate of zinc sulphide and 
barium sulphate will be obtained. This precipitate is washed 
with water, filter-pressed, dried, and then calcined at a red heat 
in a suitable furnace. While still hot the mass is thrown into 
cold water, which causes it to become rather denser than it other- 
wise would be, and thus have more body; the product is finally 
ground as fine as possible and dried, when it is ready for use. 
The calcination causes some of the zinc sulphide to become con- 
verted into oxide, and thus the final product will consist of a 
mixture, more or less intimately united, of zinc sulphide, zinc 
oxide, and barium sulphate. It is quite possible that the zinc 
compounds are in a state of more intimate union than would be 
the case if they were simply prepared separately and then mixed 
together ; that, in fact, an oxysulphide of zinc is formed. The 
exact composition of the pigment will depend upon several factors, 
the proportion between the barium and zinc salts used may be 
varied, while the duration of the calcination will influence the 
result by increasing the proportion of oxide in the pigment. 

In a later patent Orr describes a similar process, but the 
materials used are zinc sulphate and strontium sulphide. The 
patentee proposes to make the zinc sulphate by slightly calcining 
poor ores of zinc in such a way that the sulphate is formed; the 
calcined mass is lixiviated with water in such a way that a solu- 
tion of 1 '150 specific gravity is obtained. The strontium sulphide 
is prepared by calcining celestine (natural strontium sulphate) 
with charcoal ; the calcined mass on lixiviation with water yields 
a solution of strontium sulphide of specific gravity 1-060. These 
two solutions are mixed in equivalent proportions when a com- 
bined precipitate of zinc sulphide and strontium sulphate is 
obtained, as shown in the following equation : 

ZnS0 4 + SrS = ZnS + Sr S 4 

Zinc Strontium Zinc Strontium 

sulphate. sulphide. sulphide. sulphate. 

This precipitate is washed, dried, and calcined as above. The 
resulting pigment is a combination of zinc sulphide, zinc oxide, 
and strontium sulphate. The patentee specifies that the calcina- 
tion shall be continued until all sulphur vapours are given off. 
The white pigment so obtained is described as being equal in 
body to white lead, while it possesses the merit of not being 
discoloured by sulphuretted hydrogen. 

Charlton white is an excellent substitute for white lead ; its 



GRIFFITH'S PATENT ZINC WHITE. 63 

colour is good, its body is nearly equal to that of white lead, 
while it possesses the advantages of not being discoloured by 
sulphuretted hydrogen, by sulphur gases or pigments, and of 
being non-poisonous. It mixes well with all vehicles, so that it 
may be used for all kinds of painting with good results. It 
mixes with all pigments except those having lead or copper as 
their base ; these it is liable to discolour on account of the 
sulphur it contains, but with any other pigments there is no- 
change produced on mixing. 



GRIFFITH'S PATENT ZINC WHITE 

was introduced about 1876, and is made under a patent dated 
1875. The process of manufacture consists in taking " vat 
waste," a residue of the Leblanc process of alkali-making and 
exposing it to the oxidising action of the atmosphere for some 
time ; after which it is lixiviated with water, and a solution, con- 
sisting largely of calcium sulphide, with some sodium sulphide,, 
obtained. To this solution an equivalent quantity of a solution 
of zinc sulphate is added, when a precipitate of zinc sulphide, 
along with some calcium sulphate, is obtained. Barium sulphate, 
in a state of fine division, is prepared by adding a solution of 
barium chloride to one of magnesium sulphate. The zinc 
sulphide and barium sulphate are now mixed together, and, 
after being dried, are calcined until no further vapours are 
given off; the calcined mass is then ground with water, 
levigated, and dried, when it is ready for use. 

Another method of making this white is to mix together 
solutions of barium chloride and zinc sulphate, when barium 
sulphate is thrown down ; to the mixture is now added the 
" vat waste " liquor as before, which precipitates zinc sulphide. 
The two compounds are thus obtained in a much better state 
of admixture than is the case when made according to the 
last process. The mixture is calcined as above. The best 
precipitating agent is said to be " pentathionide of calcium," 
prepared by boiling 65 parts of sulphur with 35 parts of 
calcium sulphide in the state of solution, when a red liquor 
is obtained, which can be used to precipitate the zinc salts. 

A dry method of preparing the white consists in mixing 
2 parts of barium sulphate with 1 part of zinc oxide and 
10 per cent, of sulphur and 5 per cent, of sodium carbonate j 
this mixture is calcined, levigated, and dried. 

A somewhat different process, one more on the lines of Orr's 
patent, is described in a subsequent patent, taken out in 1877 



64 



WHITE PIGMENTS. 



by Griffith. According to this, barium sulphide is prepared by 
calcining barytes with charcoal, and a solution is obtained by 
lixiviation in the usual way ; to this is added an equivalent 
quantity of a solution of zinc sulphate, when a precipitate, 
consisting of a mixture of barium sulphate and zinc sulphide, 
is obtained. This is calcined in crucibles, without air being 
admitted, so that very little, if any, oxidation takes place ; 
the calcined mass is ground with water, and, after being 
washed, is dried and mixed with a small quantity of magnesia, 
which imparts to it the property of more thoroughly mixing 
with oil, and gives it a softness in working which it would 
not otherwise have. 

In the preparation of zinc sulphide whites, one difficulty 
which is encountered is that the product sometimes comes from 
the calcining furnace with a yellow tint; this, perhaps, is due 
to oxidation, which must be avoided. For this purpose Griffith 
mixes the mass, before calcining, with from J to 2| per cent, of 
ammonium chloride or ammonium sulphate, which, by being 
volatile, produces an atmosphere of ammonia round the pig- 
ment, thereby preventing oxidation and the discolouration of the 
pigment. 

A rather novel method of preparing sulphide of zinc was 
patented by Griffith & Cawley in 1879. The principle of this 
method lies in the fact that when zinc and sulphur are brought 
together, in the state of vapour, they will combine to form the 
sulphide. The process is carried out in the plant shown in 




Fig. 10. --Zinc sulphide white plant. 

Fig. 10. Sulphur is placed in the vessel, A, where it is melted; 
it flows into the egg-shaped still, B, contained in the furnace, G, 
in which it is converted into vapour; the zinc vapour passes 
into another chamber, where it comes into contact with vapour 
of metallic zinc from the still, ; the two bodies unite to form 



KNIGHT'S PATENT ZINC WHITE. 65 

sulphide, which passes along into the chambers, E, where it 
collects. These chambers are constructed similarly to those 
used in making ordinary zinc white ; but they are kept hot, so 
that any excess of sulphur vapour can pass through them and 
be collected in tne chamber, F. The zinc sulphide which 
collects in the chamber, E, is ready for use as a pigment, or 
it can be mixed with barytes, &c. D is a revolving scraper, 
for the purpose of keeping the mouth of the zinc still free from 
any deposit of sulphide which might tend to cause it to become 
chocked up. 

Griffith's patent zinc white is very largely used as a pigment ; 
its colour is good, in body it is nearly, if not quite, equal to 
that of white lead, while it is far superior to that of ordinary 
zinc white. It does not become discoloured by exposure to 
sulphuretted hydrogen, or to any other sulphur compounds ; it 
resists exposure to all atmospheric agencies, and is one of the 
most permanent white pigments known. It mixes well with 
oil, working very freely under the brush ; in this respect it is 
quite equal to white lead. It mixes with all pigments, excepting 
those containing lead or copper, without being changed by them, 
or changing them in any way. 

A sample of Griffith's zinc white, analysed by the author, had 
the following composition : 

Water, hygroscopic, .... 1*362 per cent. * 

Water, combined, 3 '7 12 

Zinc oxide, Zn O, . . . . . 9'182 

Zinc sulphide, Zn S, . . . . 23'041 

Barium sulphate, Ba S 4 , . . . 62 '541 ,, 



KNIGHT'S PATENT ZINC WHITE. 

This zinc white is prepared, according to the patent of 1876, 
by adding sulphuretted hydrogen or a solution of calcium 
sulphide or calcium pentasulphide to hot solutions of either zinc 
chloride or zinc sulphate ; the precipitate of zinc sulphide 
obtained is collected, washed, dried, and calcined. To a mixed 
solution of zinc sulphate and kieserite, which is crude magnesium 
sulphate, a solution of sodium carbonate is added, whereby a 
mixed precipitate of the carbonates of zinc and magnesia is 
obtained; this, after washing, drying, and calcining, is mixed 
with the calcined zinc sulphide obtained in the first instance. 

The best precipitant for the zinc is said to be calcium penta- 
sulphide prepared by boiling vat-waste liquor with sulphur in 
such a way that for every 40 parts of calcium there are 160 parts 

5 



66 WHITE PIGMENTS. 

of sulphur ; a red liquor is obtained which may be used for pre- 
cipitating the solutions of zinc. 

In a patent taken out in 1882 a process of preparing zinc 
sulphide white is described which is rather more complicated. 
The process consists in preparing three solutions. No. 1 is a 
solution of sodium sulphide, obtained by boiling caustic soda 
with sulphur in the usual way. No. 2 is a solution of zinc 
sulphate. No. 3 is a solution of strontium chloride prepared by 
dissolving the carbonate in hydrochloric acid. 

Nos. 1 and 2 are mixed in equivalent proportions, whereby 
there is formed a precipitate of zinc sulphide and a solution of 
sodium sulphate. To the mixture is now added an equivalent 
quantity of solution No. 3, when a precipitate of strontium 
sulphate and a solution of sodium chloride are formed. A 
precipitate consisting of zinc sulphide and strontium sulphate in 
equivalent proportions is thus obtained ; this is collected, washed, 
dried, and calcined. The calcined mass is finished by grinding, 
levigating with water, and drying. 

This white has similar properties to the two whites described 
above. 

Besides the processes described above for making zinc sulphide 
whites, the following may be mentioned as having been made the 
subject of patents: Parnell dissolves zinc in alkaline solutions 
and throws down the sulphide by means of an alkaline sulphide, 
the pigment being finished by calcining as above. William's 
process does not differ much from that of Orr; he takes a solution 
of zinc chloride and throws down the sulphide by means of a 
solution of calcium sulphide ; after separating the solution of 
calcium chloride which is formed, a solution of barium chloride 
is added and then one of sodium sulphate; the combined precipi- 
tate of zinc sulphide and barium sulphate is then finished by 
calcining ; sometimes a little magnesia is added to enable the 
pigment to mix with oil more perfectly. Glaus dissolves zinc 
carbonate (either the natural variety calamine, or the artificial 
product) in ammonia; to this is added a solution of barium 
sulphide which throws down a precipitate of zinc sulphide and 
barium carbonate. This precipitate is collected, dried, and cal- 
cined out of contact with air; while still red hot the calcined 
mass is thrown into cold water, then ground, levigated with 
water, and dried. The use of the waste liquors of galvanisers 
is mentioned as a source of the zinc solution by Glaus in his 
patent specification. 

One of the difficulties met with in the preparation of the zinc 
whites just noted is that they are very liable to turn yellow 



ASSAY AND ANALYSIS OF ZINC SULPHIDE WHITES. 67 

during the process of calcining, which yellow tint is preserved 
in the finished product. This yellow tint is obviously objec- 
tionable and is probably due to over-oxidation. Several remedies 
have been proposed to avoid this defect. One of these is to 
calcine without exposure to the air so that oxidation cannot take 
place ; another remedy is described in the notice of Griffith's 
white ; still another is not to wash completely the precipitate, 
but to leave a little of the soluble salts in it which tend to 
preserve the purity of the colour. 

The defect is not so prominent when the pigment is made as 
proposed by Knight, by mixing 1 part of zinc sulphide with 2 to 
3 parts of zinc oxide and 4 or 5 parts of barytes with a little 
magnesia ; this mixture is calcined out of contact with the air, so 
that there is little or no discolouration ; still this method does 
not give as good results as the other processes which have been 
described. 

ASSAY AND ANALYSIS OP ZINC SULPHIDE 
WHITES. 

As these are of such variable composition it is necessary to 
assay them for colour and covering power in the usual way. 

An analysis of these whites is rarely required. When such is 
the case, the analysis of Griffith's white given above, and that of 
the lithophone which is given below, will show what to look for 
in these whites. 

They are distinguished from other white pigments by being 
partially soluble in hydrochloric acid, sulphuretted hydrogen 
being given off during the solution of the pigment in the acid. 

The solution in hydrochloric acid will show the characteristic 
tests for zinc given under zinc white, and will contain any mag- 
nesia which may have been added to the white, as also any 
calcium which may be present. If the white has been made with 
barium or strontium carbonates, the solution will also contain 
barium or strontium ; these may be tested for by adding sul- 
phuric acid, when, if present, a white precipitate of the sulphates 
will be obtained, and can be differentiated by applying the dis- 
tinguishing tests. 

The residue which is left undissolved, on treatment with 
hydrochloric acid, will consist of either sulphate of barium or 
sulphate of strontium ; these may be distinguished by using the 
flame tests. 

A sample of lithophone, one of the zinc sulphide whites 
examined by the author, had the following composition : 



68 WHITE PIGMENTS. 

Barium sulphate, Ba S 4 , . . 67 '96 per cent. 
Zinc oxide, Zn O, . . . . 7 '32 

Zinc sulphide, Zn S, . . . 24 '85 

This pigment possessed a good body and worked well in oil ; it 
had a slight yellow tint. 

BARYTES. 

Barytes is one of the most important white pigments at the 
disposal of the painter ; probably, in this respect, ranking next 
to white lead, both as to the extent of its use and to its qualities 
as a pigment. In composition it is the sulphate of the metal 
barium and is obtained from both natural and artificial sources. 
The composition is indicated by the formula Ba S O 4 . Naturally, 
it occurs in large quantities very widely distributed, forming the 
mineral barytes, heavy spar, or, as it is called by the lead miners, 
cawk. Artificially, it is obtained whenever sulphuric acid or 
solution of a sulphate is added to a solution of a barium salt. 

In this country barytes is found most abundantly in Derby- 
shire ; while in Cornwall, Devonshire, Cumberland, several 
places in Wales and Ireland, it is also found and worked. It 
occurs in the form of large crystalline masses, generally opaque, 
but sometimes transparent pieces are found ; in some cases the 
crystalline character is not very strongly developed and then the 
barytes appears to be more or less amorphous. The normal 
colour of the barytes is white, but often it has a faint yellow 
tint, due to the presence of small quantities of oxide of iron ; it 
often has small streaks of lead ore (galena) distributed through- 
out its mass. 

The following analyses of two samples of Derbyshire barytes 
will show the average composition of this mineral : 

Opaque Crystalline 

variety. variety. 

Specific gravity, . . . . . 4112 5 '02 

Barium sulphate, Ba S 4 , . . . 98 '796 98 '566 

Ferric oxide, Fe 2 O s '211 '830 

Calcium sulphate, Ca S 4 , . . . "767 '504 

Magnesia, MgO, trace '017 

As a rule the transparent variety is heavier than the opaque 
variety, as is seen from the above analyses. Of two samples of 
Cumberland barytes, one, a transparent variety, had a specific 
gravity of 4-5, while the other, an opaque sample, had a specific 
gravity of 4 -3. 



PREPARATION OF BARYTES. 



PREPARATION OF BARYTES. 

In principle the preparation of barytes as a pigment is very 
simple. It merely requires grinding to a fine powder, when it 
is ready for use. In practice, however, to ensure the best 
results a little more than this is required. The crude barytes 
comes to the grinder in large lumps more or less contaminated 
with foreign minerals ; the crude material is first hand-picked to 
separate out all pieces that are not barytes and in some cases to 
pick out the small pieces of lead ore with which the mineral may 
be mixed. The subsequent treatment varies a little at different 
barytes works, according to the character of the machinery which 
is available, but the following may be taken as a description of 
the general method in use for preparing barytes : After being 
picked, the pieces or lumps of barytes are ground to a coarse 
powder under edge-runner mills ; from these it is transferred to 
flatstone mills, in which it is further ground, usually under 
water, a constant stream of which is caused to pass through the 
mills during the operation; from these mills the ground barytes 
is passed to settling tanks, which are usually constructed of 
stone ; in them the barytes settles in a few hours, the super- 
natant water is run away, while the top portion of the mass of 
barytes, being the finest, is taken and stored ready for the next 
operation; while the bottom portion, which is usually much 
coarser, is sent back into the grinding mills again to be re- 
ground. The grinding cannot be too well done as on it depends 
the quality of the resulting pigment. 

After the grinding the barytes is next subjected to a bleaching 
process to free it from any yellow tint it may possess. This tint 
is due, in almost all cases, to oxide of iron, from which barytes is 
rarely free. The process is carried out in stone cisterns, the size 
of which will vary in different works according to the quantity 
of barytes it is desired to treat at one time ; a steam pipe is fitted 
to the cistern, so that the contents may be heated, an operation 
which facilitates the bleaching. The wet barytes from the 
settling tanks is run into these stone cisterns, and heated up to 
near the boiling point ; then a quantity of hydrochloric acid is 
run into the cistern, the amount used varying considerably. As 
a matter of fact it should be proportioned to the amount of oxide 
of iron in the crude barytes, but, as a rule, it is added in a rule 
of thumb sort of way at most works; on an average about 1 cwt. 
of acid to 1 ton of crude barytes is used. 

The acid extracts the oxide of iron, leaving the barytes quite 



70 WHITE PIGMENTS. 

white. Hydrochloric acid is the best to use for bleaching barytes, 
because it has most solvent action on the oxide of iron, and 
because it is cheap ; if the barytes contains any carbonate this 
will be dissolved by the acid, but may be recovered by adding 
sufficient sulphuric acid to the solution to precipitate all the 
dissolved carbonate as sulphate ; it is doubtful whether the gain 
of barytes thus obtained is worth the expense. 

It is a good plan to give the barytes two treatments with the 
acid, using the second acid from one batch for the first acid treat- 
ment of a second batch of barytes, the second treatment being 
with fresh acid, which is used afterwards as the first acid for 
another batch ; the acid which has been used for two lots of 
barytes may then be thrown away. Working in this way a 
smaller quantity of acid is used, while the barytes is better 
bleached ; because the second lot of acid, being fresh and strong, 
has a better chance of dissolving out the last traces of oxide of 
iron than acid which is already spent and charged with that body. 

After treatment with acid the barytes is allowed to stand to 
settle, the acid liquor is poured off, and water is poured on to 
wash the barytes ; this washing being repeated once or twice so 
as to wash all traces of acid out of the barytes. This is now in 
some works subjected to a levigating process to obtain as fine a 
quality of barytes as possible, the coarser qualities being sent 
back into the grinding mills to be reground. It takes about 
three hours for barytes to settle out of the wash waters in the 
settling tanks which are used, the clear water is syphoned or 
drawn off in any convenient way, and the barytes, which is in 
the form of a stiff paste containing some 25 to 30 per cent, of 
water, is passed on to the drying stoves to be dried. 

The drying of the wet barytes is generally done in two stages. 
From the settling tanks the barytes is thrown on to the tops of 
drying flues, which flues are horizontal and run round three sides 
of a drying shed, and are connected at one end with a furnace or 
fire-place, at the other with a chimney. The flues are about 
2 feet 6 inches square, the sides being built of brick, while the 
top is formed of flagstones. On the top or tops of the flues the 
wet barytes is thrown, and it is allowed to remain until it 
becomes sufficiently dry to adhere together ; when it has reached 
this condition it is cut up into large bricks. The bricks are now 
transferred to the drying stoves. The drying stoves are large 
chambers built of brick ; in the centre is built a furnace which is 
so constructed that it may be fed from the outside. In some 
works there is built in the centre of the chamber a large globular 
shaped vessel of iron which is in connection with the furnace, so 



PROPERTIES AND COMPOSITION OF BARYTES. 71 

that it may be heated to a red heat ; this vessel, generally known 
as "the balloon," adds considerably to the temperature and, there- 
fore, to the drying power of the stove. Over the furnace and 
balloon, and around the sides of the stove, are built ranges of 
skeleton shelves of iron, on which is placed the bricks of barytes 
taken from the tops of the drying flues, and where they are 
allowed to remain until they get properly dry, which takes 
several days ; when dry they are removed from the stove, and 
the barytes is ground up in mills to a fine powder, and then 
packed for sale. 

" Floated barytes " is, or should be, a fine quality prepared by 
a process of levigation ; in other respects it does not differ from 
ordinary barytes. 



PROPERTIES AND COMPOSITION OF BARYTES. 

Barytes is a fine and rather heavy white powder, having a 
specific gravity of 4-5 to 4*75. It has a more or less crystalline 
structure, which causes it to have a slight gritty feel, no matter 
how fine it may be ground. It is quite insoluble in all acids 
and alkalies, a property, or rather properties, which distinguishes 
it from other white pigments. It is quite unaffected by any 
injurious influences, such as affect white lead, and, therefore, 
as a pigment it is the most permanent white known ; for this 
reason it has been called, and sold, as constant white, permanent 
white, &c. 

In body or covering power it is much inferior to either white 
lead or china clay, but it is better than zinc white. It mixes 
very well with oil, of which it takes about 7 per cent, to grind 
into a stiff paste ; as an oil paint it is satisfactory in use, but for 
water colours it is not so good, seeming to lose some of its body 
on mixing with water. 

Barytes consists essentially of barium sulphate, Ba S 4 , but 
commercial barytes generally contains traces of oxide of iron, 
silica, &c. The following analyses will show the general com- 
position of barytes : 

1. 2. 3. 

Barium sulphate, Ba S 4 , . . 98 '530 99 '860 99 '68 per cent. 

Iron oxide, Fe 2 3 , . . . O'OOS 0002 O'Ol 

Silica, Si O 2 , .... 0'500 0'040 0'31 

Alumina, A1 2 O 3 , &c., . . 0'96o 0'980 

It is largely used for mixing with other pigments, especially 
with white lead, many commercial samples of which contain from. 



or 1 



UNIVERSITY 



72 WHITE PIGMENTS. 

20 to 25 per cent, of barytes ; its presence in such is easily 
detected by its insolubility in acids and its tinging the Bunsen 
flame green. 

Blanc fixe or artificial barytes is obtained as a fine white 
precipitate when sulphuric acid or a sulphate is added to a solu- 
tion of a barium salt, as, for example, the sulphocyanide with 
aluminium sulphate ; the reaction is shown in the equation 

3Ba 2 (SCN) 2 + A1 2 3S0 4 = 3BaS0 4 + A1 2 (S C N) 6 

Barium Aluminium Barium Aluminium 

sulphocyanide. sulphate. sulphate. sulphocyanide. 

The sulphate precipitated is allowed to settle, the aqueous 
layer is run off, fresh water run in to wash the precipitate free 
from acid, &c., and the precipitate collected on niters. It now 
forms a pasty mass containing from 70 to 75 per cent, of barium 
sulphate, and from 25 to 30 per cent, of water, and in this form 
is largely sold to paper stainers, cotton finishers, and others. 
It is rarely dried for use as a pigment in oil and water-colour 
painting. It is much more expensive than ordinary barytes, 
because the natural barytes is the source from which all other 
barium salts are derived, and the cost of making first the chloride 
and then the sulphate from this adds to the cost of the latter. 
Barium salts, such as the acetate, sulphocyanide, &c., are used in 
the preparation of mordants, such as acetate of chrome, sulpho- 
cyanide of alumina, &c., by double decomposition with sulphates 
of chrome and alumina, in which case barium sulphate is also 
formed as a bye-product ; this may be used as blanc fixe, provided 
it is well washed. 



ASSAY AND ANALYSIS OF BARYTES. 

Barytes should be assayed for colour and covering power in 
the usual way. There being no cheaper pigment, barytes is 
never adulterated, but it is used as an adulterant in other pig- 
ments. It is distinguished by its high specific gravity, being 
heavier than any other white pigment except white lead ; and 
by its insolubility in acids, which distinguishes it from all other 
white pigments except china clay. Barytes can be detected by 
moistening a little with hydrochloric acid, and holding it on a 
platinum wire in the lower part of a Bunsen flame, when, if 
barytes is present, the latter will become tinted with a pale 
yellow green colour. It is not always easy to see this colour, as 
sometimes it only comes in flashes, while at others it is more 
persistent; much depends upon the amount of acid used. With 



GYPSUM, TERRA ALBA, MINERAL WHITE. 73 

a little care the presence of barytes in any pigment may be de- 
tected by this test. 

GYPSUM, TERRA ALBA, MINERAL WHITE. 

Gypsum is a mineral found in great abundance in many parts 
of the world. In this country it is found at Chellaston, near 
Derby, Aston-on-Trent, and a few other places in Derbyshire, at 
Newark-on-Trent in Nottinghamshire, Fauld in Staffordshire, 
and at Netherfield in Sussex. At these places it occurs in large 
quantities. In smaller quantities it is found in other places. 
In France there are large deposits, more especially in the 
district round Paris ; it is also found in Germany, America, 
Canada, and other countries. 

Gypsum is the sulphate of calcium, OaSO 4 ; but, unlike the 
corresponding barium sulphate, gypsum contains 2 molecules 
of water of crystallisation, so that it has the formula, 
Ca S O 4 , 2 H 2 O ; this water of crystallisation confers on it 
some important properties which will be briefly noticed 
presently. The composition of gypsum is 

Sulphuric anhydride, S 3 , . . . 46*51 per cent. 

Calcium oxide, Ca 0, 32 '56 ,, 

Water, H 2 O, 2093 

Gypsum is found in several forms. The most useful form is 
that in large amorphous, crystalline masses of a white or nearly 
white colour and more or less opaque; this is the variety used 
for a pigment. Often it occurs more or less coloured in a 
variegated manner ; this variety is known as alabaster, Derby- 
shire spar, and is used for making ornaments. Satin spar is a 
variety of gypsum occurring in large fibrous silky-looking pieces ; 
this variety is also used as a pigment. Selenite is a variety 
which occurs in large transparent flat pieces, which have the 
property of cleavage very highly developed ; this is chiefly used 
for optical purposes. In all its forms gypsum is a soft mineral, 
easily scratched with the finger nail and easily ground into a 
white powder. Its specific gravity varies from 2-28 to 2'33. 

For use as a pigment gypsum is ground up in the same way 
as barytes, when it is- obtained in the form of a soft white 
powder of a very good colour known as terra alba, mineral 
white, satin white, &c. 

It can also be prepared artificially. It is a bye-product in 
some operations, as in preparing acetate of alumina from 
sulphate of alumina and acetate of lime, or, generally, whenever 



74 WHITE PIGMENTS. 

acetate of lime or other soluble calcium salts are precipitated 
by sulphates 

3Ca2C 2 H 3 2 + A1 2 3SO 4 = 3CaS0 4 + A1 2 (C 2 H 3 2 )6 
Calcium acetate. Aluminium Calcium Aluminium 

sulphate. sulphate. acetate. 

In preparing many coal-tar colours gypsum is obtained, and 
this bye-product could be used in preparing the so-called aniline 
lakes (see Chapter IX.). 

The colour of gypsum is generally very good and in tone is a 
bluish-white, rather bluer than barytes, but not so blue as white 
lead ; occasionally samples may have a yellow tint, which is due 
to the presence of oxide of iron, which, however, can be 
eliminated by treatment with acids, as in the case of bleaching 
barytes. It is much lighter than either white lead or barytes, 
but is rather heavier than china clay or zinc white. Its body 
is not as good as that of white lead, but it is at least equal to 
barytes in this respect and is superior to zinc white. It mixes 
well with either water or oil, and, being neutral in its properties, 
it can be mixed with all other pigments without affecting 
them, or being affected by them in any way. On exposure 
to light and air it is unaffected, being one of the most permanent 
pigments known. It is used very largely by paper stainers and 
makers of paper hangings, who prefer it to barytes on account of 
its having more body when used for that class of work. It is 
used in the finishing of cotton goods, in paper making, and for 
a variety of other purposes where a cheap white pigment is 
required. 

Gypsum is slightly soluble in water, about 1 part in 500 ; this 
solution will give a precipitate of calcium oxalate on addition of 
ammonium oxalate and a precipitate of barium sulphate on 
addition of barium chloride. It is more soluble in hydrochloric 
acid ; long boiling with dilute hydrochloric acid will dissolve it 
without effervescence, and the solution will show the presence of 
both calcium and sulphuric acid on the application of the usual 
tests for those bodies. Gypsum is also soluble in solutions of 
ammonium salts. 

At a temperature of about 300 F. gypsum loses its water of 
crystallisation and forms a white powder, which has the property 
of combining with water and setting into a hard mass ; this 
property is a very important one, the manufacture of the white 
powder being carried on on a large scale, and the product sold 
under the name of plaster of Paris for various ornamental and 
useful purposes. 



WHITING, PARIS WHITE, SPANISH WHITE, ENGLISH WHITE. 75 

ASSAY AND ANALYSIS OF GYPSUM. Gypsum 
should be assayed in the manner given in Chapter X. for tint, 
brilliancy, covering power, and other properties appertaining 
to a pigment. A chemical analysis is rarely required and it 
can be made in the usual way ; oxide of iron, silica, barytes, 
and whiting being the substances most likely to be added or 
present in a sample of gypsum. Generally, the gypsum offered 
commercially is nearly pure, containing about 78*5 per cent, 
of calcium sulphate, the rest being water of crystallisation. 

STRONTIAN WHITE. 

Strontian White. Sulphate of strontium, Sr S O 4 , occurs 
naturally as the mineral celestine in small quantities in many 
places ; it is used on a limited scale as a pigment, being pre- 
pared for this purpose in the same way as barytes is made, for 
which it is often sent out as a substitute. It can also be made 
by adding a solution of sulphuric acid or a sulphate to a solution 
of a strontium salt. Prepared in this way it enters into the com- 
position of some patent white pigments. 

As a pigment it is quite equal, if not superior, to barytes in 
some points, and in all respects its properties are identical. If 
it could be obtained in larger quantities it would be a formidable 
rival to both barytes and gypsum. 

Strontium sulphate may be distinguished from barytes by its 
lower specific gravity and by its giving a crimson colour to the 
Bunsen flame j this latter test will also distinguish it from 
gypsum, from which it is also distinguished by its less solubility 
in water and acid and its higher specific gravity. 

WHITING, PARIS WHITE, SPANISH WHITE, 
ENGLISH WHITE. 

The body sold under all these names, of which the first is by 
far the most common, is the carbonate of calcium, Ca C O 3 . 
This compound is one of the most important and abundant of 
rock-formers ; it occurs in large masses, forming, in many cases, 
mountain ranges of no mean size, and, in a variety of forms, 
only surpassed in number by those of quartz. It not only 
occurs as a rock-former, but it is found as a mineral, that is 
as a body of definite form and composition, in great abundance 
and variety of shape. One of the rocks made of calcium 
carbonate is chalk, which, in this country, is largely deve- 
loped both north and south of London, especially in the 



76 WHITE PIGMENTS. 

county of Sussex, the coast line of which is celebrated for its 
chalk cliffs. It is also found in the north of France, in the 
district between Paris and the English Channel. In England 
it is extensively quarried for a variety of purposes, one of 
which is the manufacture of whiting ; and English whiting has 
a world-wide reputation for quality. Chalk is essentially a rock 
of organic origin, and it owes its formation to the operations 
of certain minute organisms known as Foraminifera ; these bodies 
have the property of secreting a shell of calcium carbonate, and 
chalk is the remains of a deposit of the shells which has been 
formed after the death of the animal that inhabited them. 
Chalk is very nearly pure calcium carbonate, but it contains 
traces of silica, oxide of iron, alumina, c. Besides the traces 
in which silica is found distributed throughout the main mass 
of the chalk, it also occurs in large masses of very irregular 
shapes known as flint. 

For use as a pigment, the chalk is quarried, ground under 
water in edge-runner mills to a coarse powder, then passed to 
flatstone mills, where it is ground very fine ; from which mills 
it is run into tanks, in which the coarser and heavier grit and 
sand settles, while the finer chalk passes on to other .tanks in 
which it settles. When the settling tanks or pits are full, the 
chalk or whiting is dug out and dried. It is first taken to a hot 
room, on the floor of which it is thrown in heaps. Under the 
floor are a number of flues connected with a furnace fed from 
the outside of the chamber; sometimes the flues do not pass 
under the floor of the chamber, but round three sides of it, much 
in the same manner as in the drying-room used in making 
barytes. In this room it remains about twelve hours, when the 
great bulk of the water it contains will have evaporated away. 
The partially dry mass is now cut into masses of a cubical 
shape, and taken to a large drying-room, constructed similarly 
to the one described under barytes. In this hot chamber it is 
placed on iron racks, and kept there until it is thoroughly dry, 
which will take from 24 to 48 hours, according to the degree of 
dry ness of the whiting, and to the temperature of the drying 
stove ; the drying is also facilitated if a current of air is sent 
through the drying-room during the process, as is done in some 
works. The temperature of the drying stove should not be too 
high, and in arranging the whiting on the racks it is advisable 
not to place it in any position where it is liable to be subjected 
to extreme heat. After being dried, the whiting is ground in a 
flatstone mill before it is sent out for use. 

It may be stated that one reason for avoiding over-heating 



COMPOSITION AND PROPERTIES OF WHITING. 77 

during the process of drying is to prevent the conversion of 
the carbonate of lime into quicklime. One of the properties 
of carbonate of lime is that when heated it loses its carbonic 
acid, and passes into the state of oxide, which has some powerful 
alkaline properties undesirable for some purposes to which whit- 
ing is put. 

Paris white is a finer quality of whiting, prepared from chalk 
in the same way, but the grinding is more thoroughly done. 
Spanish white is an old name given to Paris white sold in a 
cylindrical form, prepared by moulding the wet material into 
that form, and allowing it to dry in the open air. 

COMPOSITION AND PROPERTIES OF WHITING. 
Whiting is a dull white powder of an amorphous character, 
and soft to the feel ; it is moderately heavy, the specific gravity 
being about 2 -5 to 2*8. It consists chiefly of calcium carbonate, 
but it may also contain traces of silica, water, &c. The following 
analysis of an ordinary commercial sample will show the average 
composition of whiting : 

Calcium carbonate, Ca C Os, . . 94*795 per cent. 

Silica, Si 2 , 3 '030 

Water, H 2 0, 2'175 

It is quite insoluble in pure water, but in water containing 
carbonic acid gas in solution it is soluble. It is also soluble in 
acids such as acetic, hydrochloric, nitric, &c., with effervescence 
due to the evolution of carbonic acid gas. Sulphuric acid de- 
composes it with effervescence, carbonic acid being evolved, and 
insoluble calcium sulphate formed. There are but few white 
pigments which are soluble in acids with effervescence ; white 
lead and whiting are the most common, while the carbonates 
of barium, strontium, and magnesium are occasionally met with. 

Heat decomposes it with the evolution of carbonic acid gas, 
C O 2 , and the separation of calcium oxide, quicklime, Ca ; this 
property is taken advantage of in preparing lime for making 
cements, mortars, tkc. 

As a pigment it is mostly used as a body colour in distemper 
work, colouring walls, ceilings, tfec., using water as a vehicle. 
It is not used as an oil colour, for it is subject to the defect that, 
when mixed with oil, it loses its white colour and burns a dirty 
grey; mixed with about 18 per cent, of linseed oil, it forms the 
useful article known as putty. 

It has a tendency to be somewhat alkaline in its properties. 
In this respect much depends upon the care which has been 
exercised in drying the whiting during the process of manufac- 



78 WHITE PIGMENTS. 

tare ; over-heating is apt to cause the formation of small quanti- 
ties of quicklime, which increases the alkaline properties of the 
whiting. It is not safe, therefore, to mix it with such pigments 
as Prussian blue, chrome yellow, verdigris, emerald green, <fec., 
which are more or less affected by alkalies. Most other pig- 
ments ultramarine, yellow ochre, oxide reds, &c., are not 
affected by whiting in any way. 

It is quite permanent when used as a pigment, and is not at 
all acted on by any of the ordinary atmospheric agents which are 
often destructive to pigments. 

ASSAY AND ANALYSIS OF WHITING. The assay 
and analysis of whiting is a matter which is rarely required, 
the pigment is so cheap that it is not subject to any adultera- 
tion. When, however, an assay is required, it can be done in 
the usual way. 

Whiting is distinguished by the following tests : It is soluble 
in dilute hydrochloric acid with much effervescence ; there may 
be left undissolved small traces of silica in a gelatinous form, 
but no considerable amount of white powdery insoluble residue 
will be found. On adding to this solution ammonium chloride 
and ammonia, no precipitate of iron or alumina should be ob- 
tained, or, at most, a very slight one. On further adding 
ammonium sulphide, no precipitate should be obtained. Addi- 
tion of ammonium oxalate to the same solution throws down a 
white precipitate of calcium oxalate. The nitrate from this 
should not give with sodium phosphate more than a mere trace 
of a precipitate of magnesium phosphate. 

The quantity of calcium carbonate or whiting which may have 
been added to any pigment may be ascertained by throwing it 
down as calcium oxalate, as described above, and then filtering 
off this precipitate but it is best to allow it to stand for some 
time before filtering, so as to give time for the precipitation to 
become complete. The precipitate is well washed, then dried, 
placed in a weighed crucible, burned, and weighed in the usual 
manner. The weight of the residual calcium carbonate gives the 
weight of the whiting in the sample taken. 

MAG-NESITE 

is the form in which magnesium carbonate, Mg C O 3 , occurs in 
many localities, closely resembling the more crystalline varieties 
of calcium carbonate in appearance. Ground up and levigated 
with water it has been offered, from time to time, as a pigment, 
but it has not come into general use. It is rather heavier than 



CHINA CLAY. 79 

whiting, its specific gravity being about 3-05. Its colour is 
good, and it forms a dense white powder ; in body and coverin^ 
power it is quite equal to barytes. It works well in both oil 
and water, and mixes fairly well with other pigments, although 
it has a slight tendency to possess alkaline properties. It is 
quite permanent, and resists exposure to light and air. 

Magnesite may be distinguished by the following tests : 
It is soluble in dilute hydrochloric acid, or in sulphuric acid 
with effervescence. This solution gives no precipitate with 
ammonium chloride and ammonia, ammonium sulphide or 
ammonium oxalate, but gives a copious white, somewhat 
crystalline, precipitate of ammonium magnesium phosphate on 
addition of sodium phosphate to an ammoniacal solution; this 
precipitate, on ignition, is converted into magnesium pyro- 
phosphate. 

CHINA CLAY. 

China clay or, as it is sometimes called, kaolin, which is its 
Chinese name, is a natural product, and only requires levigating 
and drying to prepare it for use as a pigment. It owes its name 
to its use in the manufacture of the white and finer varieties of 
pottery. Prior to its discovery in this country the pottery in 
common use had a brown colour, due to the use of clays highly 
contaminated with iron, and much of the best kinds of white 
pottery were imported from China ; hence the use of the term, 
"china," for white pottery. When the existence of the deposits 
of china clay in Cornwall was found out, and its use in making 
pottery became general, the goods so made were denominated 
"china," and the material "china clay." As a pigment it is 
nob used to the extent its many good qualities entitle it to. 

ORIGIN AND COMPOSITION OP CHINA CLAY. 
In England and other countries where china clay is found it 
is invariably associated with granite rocks, and is evidently a 
decomposition product from them. The exact cause which has 
led to the decomposition of the granite rocks in those places 
where china clay is found, and to its not undergoing this 
decomposition in others, where it is equally well developed, is 
a matter which at the present moment is one of uncertainty. 

Granitic rocks are formed by the aggregation of three minerals 
in various proportions ; these three minerals are quartz, felspar, 
and mica. Quartz is the special form of silica (oxide of silicon, 
Si O 9 ) which occurs in these granitic rocks ; it is absolutely 
unchangeable by any amount of exposure to the atmosphere. 



80 WHITE PIGMENTS. 

In granites it forms the base mass throughout which the other 
minerals are distributed. 

Mica is a double silicate of potassium and alumina, having 
usually the composition shown in the following analysis : 

Silica, Si 2 , . . . . 46 '3 per cent. 
Alumina, A1 2 3 , . . . 36 '8 ,, 
Potash, K 2 0, . . . 13-7 

but the proportions vary in micas from different districts ; in 
some varieties of mica the potash is replaced by magnesium, in 
others by sodium. Mica is characterised by crystallising in thin 
transparent flakes or plates, which are sometimes found of large 
size; these flakes have the property of cleavage very highly 
developed, so that mica can be easily split up into very thin 
leaves. It is a very refractory substance, and will resist a 
considerable amount of exposure to atmospheric influences 
without being decomposed ; it will resist heat to a great extent. 
During the natural decomposition of the granitic rocks, which 
gives rise to the formation of china clay, the mica and quartz 
undergo no change whatever. 

The third mineral which is found in granite is felspar ; this 
occurs in the rock in the form of white or slightly reddish, 
opaque rhombic crystals. In some granites the crystalline 
form of the felspar is well developed, while in others it assumes 
an amorphous appearance. There are many varieties of felspar, 
but they are all of them double silicates of alumina and of the 
alkali-metals, while in some calcium replaces to a greater or 
less extent the alkali-metal. The most important and most 
common felspar is the potash variety, known as orthoclase; this 
has the composition shown in the following analysis : 

Silica, Si0 2 , . . . 64 -20 per cent. 
Alumina, A1 2 Oa, . . . 18 '45 ,, 
Potash, K 2 O, . . . 17'35 

On comparing this analysis with that of the mica given above it 
will be seen that the felspars contain a larger proportion of silica 
and of the alkaline constituent, while the proportion of alumina 
is only about one-half that found in the micas. 

The formula of felspar is 6 Si 2 , A1 2 O 3 , K 2 O. 

The soda-felspar has a similar composition ; in some felspars 
both alkali-metals are present. 

Felspar is capable of undergoing decomposition when exposed 
to the destructive action of the atmosphere, which decomposition 
is probably chiefly brought about by the action of the carbonic 



MANUFACTURE OF CHINA CLAY. 



81 



acid and water which are always present in the atmosphere ; 
probably the alkali is eliminated in the form of carbonate, which, 
being soluble in water, passes into solution in the springs which 
rise in the granite, while the silica and alumina are left behind 
in an insoluble form, the kaolin or china clay of the potter. The 
average composition of china clay is : 



Silica, Si0 2 , . 
Alumina, A1 2 3 , 
Water, H 2 0, . 



47 per cent. 

40 

13 



which corresponds to the formula, 2 Si O 2 , A1 2 3 , 2 H 2 0. But, 
as a rule, the composition of the china clay found in various 
localities varies somewhat from this, as will be seen in the 
following analyses of some china clays from several widely- 
separated localities, which are quoted from several sources. 

ANALYSES OF CHINA CLAYS. 



CONSTITUENT. 


_: 
1 
1 


4 


f| 

S 


A 


1 


^ 




s 


a 


50 


a 


S 


g 




6 


a 
O 


J4 


& 


Q 


8 


Silica, Si 2 , . 
Alumina, A1 2 63, 


46-78 
39-60 


50-50 
33-76 


47-13 
36-76 


48-37 
34-95 


47-20 

38-80 


46-32 
39-72 


Water, H 2 0, . 


13-16 


11-22 


15-13 


12-62 


12-00 


12-67 


Potash, K 2 0, . 


0-12 


1-90 




2-4 


1-76 


0-44 


Ferric oxide, Fe 2 3 , 


0-09 


1-80 


trace. 


1-26 




0-27 


Lime, Ca 0, . 


0-07 




0-04 




0-24 


0-36 


Magnesia, Mg O, 


... 


0-80 








0-36 



China clay occurs in large deposits along with the other con- 
stituents of undecomposed granite, the china clay usually forming 
from 15 to 20 per cent, of the whole deposit. The deposit does 
not usually occur in layers, but in basins in the surrounding 
granite ; over the deposit there is usually a layer of soil known 
locally as the " overburden," which varies in thickness from one 
to several feet. Fig. 11 is a drawing of a china-clay deposit 
taken from Mr. David Cook's Treatise on China Clay. The 
deposits of china clay are often of great depth. 

MANUFACTURE OP CHINA CLAY. The process of 
extracting the china clay from the undecomposed quartz and 
mica is essentially one of levigation and is comparajiwsel^&imple. 

s^$i 

(UNIVERSITY) 



82 



WHITE PIGMENTS. 



When a deposit of china clay has been found, the overburden 
is removed and usually two shafts are sunk ; one of these is as 
close as possible to the edge of the deposit and is known as the 
permanent shaft, the other is sunk in the centre of the deposit 
and is known as the " rise," partly because it is often not sunk 
from the top ; but the permanent shaft is first sunk and then a 




Fig. 11. China-clay works. 

drift is dug to the centre of the deposit and the centre shaft dug 
out from the bottom ; in any case, whether the rise is sunk from 
above or dug out from below, a drift or horizontal shaft connects 
it with the permanent shaft. The size of these shafts varies in 
different places, but it is not necessary that they should be of 
large dimensions ; the rise in particular need not be any larger 
than can comfortably be excavated. 

The permanent shaft is fitted with pumps, the use of which 
will be seen presently. 

The rise is fitted with a square wooden pipe of 9-inch side 
and long enough to reach from the top of the rise to the 
horizontal drift at the bottom. In one side of this tube is a 
number of holes about 4 inches in diameter and at a distance of 
1 foot from centre to centre ; before placing in position these 
holes are stopped up by plugs known as " buttons," and the tube 
itself is known as the " button-hole launder"" launder " being a 
local term to signify any tube or trough through which liquors 
or materials can flow. After the button-hole launder is placed 



MANUFACTURE OF CHINA CLAY. 83 

in position in the rise all the space between it and the sides of 
the rise is filled up with clay pressed down rather tightly. 

One essential feature of china-clay works must be a plentiful 
supply of water; without this it cannot be profitably carried on, 
and the source of supply should be, if possible, above the works, 
so that it may be supplied to any part by the aid of gravity 
alone ; if it be on a level or below the works, then pumping 
becomes a necessity, and this should be avoided whenever pos- 
sible, as it is a source of expense which reduces the profit of 
working the deposit. 

A china-clay works is shown in Fig. 11, in which the relative 
position of the clay deposit with the surrounding rocks is shown, 
with the overburden, the permanent shaft, the button-hole 
launder, and the pumping house. 

The deposit of china clay is worked in the following manner: 
The overburden is removed and then a current of water is 
directed against the sides, known at the works as the " stopes * of 
the deposit; naturally these currents of water will wash the china 
clay, &c., to the lowest part, which is the centre of the deposit. 
At the bottom of the stopes is arranged a hollow or sand pit ; in 
this the sand or quartz collects, and is removed from time to time 
by means of a sand waggon which is hauled up the sides of the clay 
pit on an inclined tramway by means of a windlass driven by the 
engine. The clay along with the mica flows through the topmost 
hole in the button-hole launder into the horizontal drift at the 
bottom, from thence it passes into the permanent shaft, from 
which it is again pumped to the surface to be treated in the 
manner presently to be described. As the level of the china-clay 
deposit descends, the buttons in the launder are removed until, 
finally, when all the clay is worked out, the bottom of the 
deposit is reached, and with it the last hole in the launder. The 
length of time required to work out a deposit depends entirely 
on its extent, which may be small or large ; the deposits, as a 
rule, are large and take some years to work out. 

The crude clay as it comes from the pits or stopes is still 
impure ; it contains much fine sand or quartz, most of the mica 
(which does not separate so readily out of the clay as the sand 
does), together with undecomposed particles of felspar ; from 
these it requires to be separated ; this is done by a process of 
levigation by means of water. In early times when the clay was 
first worked this was done in a crude form, usually in a series 
of tubs, but now a better system is adopted, which is shown in 
Fig. 12. 

From the button-hole launder the water containing the clay, 



84 



WHITE PIGMENTS. 



sand, mica, &c., is pumped into a series of troughs of wood,, 
generally known as "launders;" these are long and wide; across 
these at intervals are placed a number of pieces of wood, called 
the u drags," which serve to impede the flow of the water, and 
cause it to form a series of pools, in which the heavier particles 
of sand can collect. These launders are emptied of the sand 
which accumulates in them from time to time, so as to give- 



Waste Mica 




CLAY LI NH AY 

t'M'M t'M'/A V/,'/,>},\ VM'M'A 

Fig. 12. China-clay works. 

plenty of room for the sand to deposit. From the launders the 
water, with the clay and mica, passes on to what are called the 
" micas," an ingenious arrangement for promoting the deposition 
of the light and flaky pieces of mica. The micas are a series of 
troughs 20 feet long, 2 feet wide, and 6 inches deep, placed side 
by side in a peculiar manner. The clayey water from the launders 



MANUFACTURE OF CHINA CLAY. 85 

first passes into two of these, then from these into three, then 
again into four, and, finally, into six of these mica troughs ; 
thus as it passes to the exit end the flow of water is spread over 
a larger surface, and becomes more feeble, a condition which 
facilitates the deposition of the mica. The micas get filled in 
about eight hours, when they are flushed of their contents with 
water, which carries the deposited mica through suitable channels 
into the waste mica pits. 

Next in order to the micas is a set of settling pits. These are 
usually three in number, sometimes more, according to the 
quantity of clay which is being worked. These pits may be of 
any shape, but, as a rule, they are made circular (or rather 
cylindrical) in form, 7 feet in diameter, and 40 feet deep. Into 
one of these pits the clayey water from the micas is run until it 
is full, when the current is changed and the water run into the 
second one until it is full. While the filling of the second pit 
is proceeding, the clay in the first one is settling, and, probably, 
by the time that the second is quite full, has completely settled. 
The current of clayey water from the micas is now diverted into 
the third pit, while the clay in the second one is settling. The 
water in the first pit is now run or pumped off, and is generally 
used over again for washing the clay from the stopes. From a 
pit full of clayey water there will usually be obtained a deposit 
of clay about 5 feet in depth, which still contains a large pro- 
portion of water ; in such a thickness of clay, and 7 feet in 
diameter, there will be something like 285 tons of dry clay. The 
clay in the first pit is dug out and thrown into what are called 
clay tanks, where a further settling takes place ; when all the 
clay has been dug out the pit is ready to be filled again with 
water from the micas. This alternation of filling, settling, and 
emptying is carried out with the three pits in succession, so that 
it will be seen that for continuous working a series of not less 
than three pits is required ; if more pits are used, then the time 
of settling can be lengthened, which would have the advantage 
of giving a drier clay and shortening the subsequent operations. 

From the settling pits the still wet clay passes to the settling 
or clay tanks ; these are, at least, three in number, corresponding 
with the three settling pits; in some works there are more; much, 
of course, depends upon the quantity of clay it is desired to turn 
out. These settling tanks are usually rectangular in shape, about 
60 feet long by 7 feet wide and 6 feet deep, and they will hold 
about 1000 tons of clay ; in these tanks settling occurs, and the 
clay begins to assume the consistency of lard ; when this happens 
no "more clay is sent into it from the pits, and the clay in the 



86 WHITE PIGMENTS. 

tank is allowed to settle. The water is then run off, and the 
clay transferred to the drying place, where it is dried ready for 
sale. 

The clay in the clay tanks contains about 50 per cent, of water, 
most of which must be driven off before the clay is marketable. 
This drying operation is done on a series of flues, technically 
known as the "dry;" a usual size is 60 feet long by 13^ feet 
wide. In this kiln or dry there will be three fireplaces, two at 
one end, and one at the other, each fireplace having three flues 
about 9 inches wide ; the sides are formed of brickwork, but the 
bottom is usually made of sand, partly because sand is a bad 
conductor of heat, and partly because any water which may 
drain through from the top of the flue readily sinks into it and 
drains away. The tops of the flues forming the bed of the dry 
is made of fireclay bricks about 18 inches wide. On these fire- 
clay bricks the wet clay from the tanks is thrown, and it remains 
until it is dry. It takes about 1 ton of coal to dry 10 tons of 
clay. 

After being dried on the dry the clay is thrown on the floor 
of the clay linhay, which is a storage place for the dry clay, from 
whence it is sent out as required. 

The dry and the linhay are parts of one large room, being 
covered over with a roof, as is seen in the drawing (Fig. 12). 

COMPOSITION AND PROPERTIES OP CHINA 
CLAY. China clay is essentially a hydrated silicate of alu- 
mina, as has been already stated ; but there are some minor 
differences in the composition of samples from various localities, 
as will be seen on examining the table given on p. 81 ; these 
are, of course, primarily due to differences in the composition of 
the granite from which the china clay has been formed, and, 
secondarily, to the degree with which the decomposition has 
proceeded. 

China clay, or kaolin, is a fine, white, amorphous powder 
having slight adhesive properties and adhering to the fingers 
when moist. 

It is light, its specific gravity being about 2*2; so that it is 
the lightest of all the white pigments. The best qualities have 
a very soft unctuous feel; the common qualities are rather 
rougher, but none have the slightest trace of grittiness about 
them. The best qualities have a pure white tint, others a more 
or less yellowish tint, which the china-clay makers are accus- 
tomed to disguise by adding a small quantity of ultramarine. 

It is quite insoluble in water, dilute acids, and alkalies. 
Boiling in strong sulphuric acid for some time decomposes it 



WILKINSON'S AND PATTINSON'S WHITE LEAD. 87 

with the formation of a gelatinous residue of silica and a solu- 
tion of alumina sulphate. Hydrochloric acid has little action 
on it. 

As a pigment it is quite permanent, resisting perfectly 
exposure to the atmosphere and to light for any length of 
time. As a pigment it is not, however, much used. In oil it 
loses its body and becomes more or less transparent. It can be 
used in water-colours and in distemper work with good results, 
and it is used in paper-making and paper-staining. It also 
finds a use in the preparation of the aniline lakes, especially 
when these are to be used in paper-staining. 

Its principal uses are for making pottery, ultramarine, finish- 
ing cotton cloths, making paper, &c. 

ASSAY AND ANALYSIS OF CHINA CLAY. China 
clay can be assayed for colour or tint, covering power, &c., 
by the methods given below. An analysis is rarely wanted, 
since it is never adulterated, while for all pigment purposes 
absolute chemical purity is not required. 

WILKINSON'S WHITE LEAD AND PATTINSON'S 
WHITE LEAD are oxy chlorides of lead prepared in various 
ways; neither pigment is now used. 

Wilkinson's white was patented in 1799, and is made by 
digesting litharge with a solution of salt until it acquires a 
pure white colour. Unfortunately, as it is the product of various 
operations, there is a lack of uniformity in its composition, which 
is much against its use as a commercial article. Another method 
of making it is to precipitate acetate of lead with hydrochloric 
acid, and to digest the precipitate of lead chloride obtained with 
basic lead acetate. 

Pattinson's lead was made by treating chloride of lead with 
lime, when it forms the basic chloride, a white insoluble body 
having a fair body, but wanting in uniformity of composition. 



88 



CHAPTER III. 
BED PIGMENTS. 

THIS is a fairly numerous and important class of painters' colours. 
They are derived from both inorganic and organic sources, and 
include some of the most highly valued and most used of the 
pigments at the disposal of the painter and artist. 

VERMILION. 

Vermilion has been used for a long time as a pigment. It 
is a compound of mercury and sulphur in the proportion of 
200 parts of the former to 32 of the latter ; its chemical name is 
mercuric sulphide, and it has the formula, Hg S. It is found 
naturally in large quantities as the mineral cinnabar, especially 
in Spain ; but it rarely occurs naturally of sufficient brightness 
to be used as a pigment, and is, therefore, mostly made artificially. 
When a current of sulphuretted hydrogen is passed through a 
solution of a mercuric salt a black precipitate of the mercuric 
sulphide, identical in composition with vermilion, is obtained ; 
this precipitate is characterised by being insoluble in most single 
acids, but soluble in a mixture of hydrochloric and nitric acids. 
By heat it is volatilised, and the sulphide sublimes in the form 
of a red powder ; this transformation from black to red can also 
be brought about by boiling it for some time with aqueous solu- 
tions of the caustic alkalies or of alkaline sulphides. What the 
cause of the change may be is rather uncertain ; probably there 
has been a re-arrangement of the atoms in the molecule of mer- 
curic sulphide ; there are many cases known of similar differences 
in the colour of inorganic compounds, as, for example, cadmium 
sulphide and basic chromate of lead. Although it is generally 
considered that the molecule of each of these poly-coloured 
bodies is always made up of the same number of atoms, yet 
there is no direct evidence on that point ; and it is quite 
possible that in the different modifications of these bodies the 
number of atoms may vary and, therefore, be arranged dif- 



MANUFACTURE OF VERMILION. 89 

ferently. This subject requires further investigation before the 
point can be definitely decided. 

MANUFACTURE OP VERMILION. Vermilion can be 
made both by dry and wet methods; the former are those 
mostly used as they give the best product; the latter are 
employed in some places but not to the extent of the dry 
methods. The product is not quite equal, although very little 
inferior to that made by the dry methods. The Chinese have 
long been renowned as makers of vermilion; their product is 
finer and more brilliant in tone than that made in Europe. 
Until lately, the process by which Chinese vermilion was made 
was not known with certainty, although it was conjectured that 
the wet method was used, and, consequently, this method is 
usually described in text-books as " The Chinese method ; " but 
this is now known to be erroneous, and that Chinese vermilion 
is made by a process very little different from that used in 
Europe. The difference in quality almost entirely arises from 
the greater care the Chinaman takes in making it. 

DRY METHODS 1st. Dutch Process. This is the 
method commonly used for making vermilion. It is conducted 
in two stages. In the first stage 108 Ibs. of mercury are mixed 
with 15 Ibs. of sulphur in a shallow iron pot; this is usually 
placed over a furnace so that a gentle heat may be applied ; the 
two bodies gradually combine together to form a black sulphide 
of mercury or "ethiops" as it is called, the union being pro- 
moted by a continual stirring with an iron spatula. When the 
combination is considered by the workman to be complete, the 
iron pot is emptied of its contents into a store pot and a fresh 
mixing is made. The "ethiops" contains some free mercury, 
free sulphur, as well as sulphide; the proportions will vary 
according to the length of time the operation has been continued, 
the heat applied, <fec. 

The second stage consists in heating black ethiops in a suitable 
furnace, whereby it is converted into the red vermilion. A 
number of simple furnaces or fireplaces are built side by side 
to form a range ; in each of these fireplaces is placed a cylin- 
drical earthenware pot, so arranged that the lower two-thirds 
of the pot are in, while the upper third is outside the furnace. 
The pots are fitted with a closely-fitting iron lid, in the centre 
of which is a small charging hole. The fire in the fireplace 
is lighted, and, when the pot has been heated to a red heat, 
a small quantity of the black ethiops obtained in the first stage 
is charged into the pot; much of the sulphur in the ethiops 
burns off; when there is no further appearance of sulphur fumes 



90 RED PIGMENTS. 

from the pot more ethiops is added ; these additions are con- 
tinued at intervals for thirty-six hours, the cover being kept on 
during the whole of the operation ; then the pots are allowed to 
cool down ' } when cold the cover is removed and the vermilion is 
found as a crust on the under side of the cover and arouDd the 
sides of the upper portion of the pot. This crust is carefully 
removed, the red portions being placed on one side for further 
treatment, while any black, unchanged portions are mixed with 
some fresh ethiops to be again heated. The red vermilion 
is now ground up as fine as possible with water; if not of 
sufficiently brilliant colour it may be treated either with acids 
or alkalies as is described below, well washed with water, 
allowed to settle out of the wash waters, dried at a gentle heat 
and sent into the market ready for use. 

2nd. Chinese Method. A few years ago a description* of 
the process used by the Chinese for the preparation of vermilion 
appeared in several journals, and at the Colonial and Indian 
Exhibition held in 1886 there was shown in the Hong Kong 
Court a Model of a Chinese vermilion factory. Like the Dutch 
method, the Chinese process is in two stages, and is carried out 
as follows : 

An iron pan, measuring 25 inches in diameter and 6 inches 
deep, is placed over a charcoal fire; into this pan is placed 
17J Ibs. of sulphur and 37J Ibs. of mercury ; heat is applied, and 
the mixture stirred until the materials melt and become amal- 
gamated together ; then 37 J Ibs. more mercury are added, and . 
the heating and stirring continued until the two bodies have >f 
become united. The pot is now removed from the fire and 
water added in sufficient quantity to form a paste, which has 
a blood-red colour ; the first stage of the process is now com- 
plete. 

Second Stage. The crude vermilion obtained in the first 
stage is broken up into small pieces and placed in iron pans 
measuring 29J inches in diameter and 8J inches deep ; on the 
top of the vermilion is placed a number of broken pieces of 
porcelain plates arranged in the form of a dome; over all is 
placed the pan used in the first stage, the two pans being luted 
together with clay, and a few vent holes left in the luting. The 
pans are placed on a furnace, which is constructed in a simple 
manner ; usually a number are built side by side. The pans are 
heated for 18 hours at a dull red heat, after which they are 
allowed to cool down ; when cold, the pans are opened, when 

* Oil and Colourman's Journal, 1883, p. 86. Chemical News, 1884, vol. 
50, p. 77. 



MANUFACTURE OF VERMILION. 91 

the vermilion is found as a red sublimate on the under side of 
the porcelain plates and the upper pan ; this red mass is 
collected and transferred to another place for the finishing 
operation. The crude vermilion which has been scraped off 
the porcelain plates is now ground as finely as possible with 
water in a mortar ; the ground colour is next mixed with water 
in which alum and glue in the proportions of 1 oz. of each in a 
gallon of water have been dissolved, and allowed to stand for 
a day ; it settles down and is found as a cake at the bottom of 
the vessel, which is made of earthenware, and has a capacity 
of 6 gallons. The top of the cake is of fine quality; this is 
separated from the bottom portion, which is re-ground up with 
the next batch ; sometimes the top portion is re-ground. After 
being washed well with clean water, the finely-ground vermilion 
is dried and then packed up ready for sale. 

WET METHODS 1st. Common Method. In making 
vermilion by this method 68 Ibs. of sulphur and 300 Ibs. 
of mercury are mixed and ground together until they are 
thoroughly incorporated ; they are then added to a solution 
of 160 Ibs. of caustic potash in water, placed in iron pots and 
heated to a temperature of 45 C., which is maintained for some 
hours. For the first two hours the water lost by evaporation is 
made good, but after this no further addition is made, and the 
mass is kept constantly stirred. After some time the mass, 
which has at first a blackish appearance, turns brown and then 
gradually passes into red j when it is considered that the colour 
is fully developed the mixture is removed from the fire, well 
washed in water and dried. This process requires careful 
watching. 

2nd. Firmenich Process. The process described by Fir- 
menich * consists in taking 10 parts of mercury and agitating 
them with 2 parts of sulphur and 4J parts of potassium penta- 
sulphide (prepared by heating potassium sulphate with charcoal) 
and boiling the residue with excess of sulphur for three to four 
hours, when it takes a brown colour ; it is then kept at a 
temperature of 45 to 50 C. for three to four days, being 
agitated at intervals during that period ; it is next treated with 
water, then with a weak lye of caustic soda (to free it from excess 
of sulphur), washed thoroughly and dried. 

In these wet processes it is important that care be taken not 
to heat the mixtures of mercury, sulphur, and alkali to too high 
a temperature ; from 45 to 50 C. is high enough. Time, not 

* Chemical News, vol. v., 1862, p. 247. 



92 RED PIGMENTS. 

heat, seems to be the most important element to consider in these 
processes ; too great a heat turns the vermilion brownish. 

The brilliancy or fire, as it is sometimes called, of the vermilion 
may be increased during manufacture by 

1st, Grinding very fine and levigating ; 

2nd, By warming with a caustic soda lye ; 

3rd, By treatment with nitric acid ; 

4th, By treatment at about 50 C. with a mixture of the 
caustic and sulphide of potash ; and 

5th, By treatment with hydrochloric acid. 

Any of these, or a combination of them, may be, and are, used 
for this purpose. 

PROPERTIES OF VERMILION. Vermilion is a very 
bright scarlet powder. It is the heaviest pigment known, 
its specific gravity being 8'2, which causes it to settle readily out 
of paints, &c., in which it is used, and renders its application 
somewhat troublesome. It is very opaque, and, consequently, 
has great covering power or body. It is quite insoluble in 
water, alkalies, and any single acid, but a mixture of nitric and 
hydrochloric acids dissolves it with the formation of a colourless 
solution of mercuric chloride j as a rule, very few substances are 
capable of acting on vermilion. 

Heated in a tube out of contact with air, vermilion first 
turns brown, then sublimes in the form of a red sublimate. 
Heated in contact with air, vermilion burns with a pale blue, 
lambent flame, giving off vapours of sulphur dioxide and mercury 
oxide j if pure, there will be but a trace of ash left ; thus a 
sample of good vermilion analysed by the author contained 

Sulphide of mercury, .... 
Ash, 



100-00 

This forms a reliable test for the adulteration of mercury, for 
any adulterant which may be used will be left behind on heating. 
The usual adulterants employed are red lead, oxide of iron, red 
lakes, vermilionettes, &c. The presence of any of these is easily 
ascertained by the application of the characteristic tests, which 
will be found described under each particular pigment. 

When used as an oil-colour vermilion is permanent ; when used 
as a water-colour it is generally considered to be permanent, but 
the experiments recently made by Capt. Abney and Dr. Russell 
throw some doubt on this point ; they found that vermilion used 
as a water-colour turned brown after two years exposure to light 



MANUFACTURE OF RED LEAD. 



93 



and air, probably owing to an intermolecular change; much 
appears to depend on the care with which the vermilion has 
l)e en made. 



RED LEAD. 

This valuable red pigment has been known and used for a 
very long time. Pliny, in his writings, describes this body, 
which in his time was known as minium, under which name 
it is also frequently referred to in later writings. Pliny also 
mentions its use for adulterating vermilion. Davy, who had 
an opportunity of examining the contents of some pots of colour 
found in the remains of Roman and Greek cities, frequently 
found red lead among them. How it was made by the ancients 
is not definitely recorded. 

MANUFACTURE OF RED LEAD. There is only one 
process for making red lead, which consists of two stages the 




Fig. 13. 

first stage has for its object the conversion of metallic lead 
into monoxide of lead; in the second stage this oxide is 
converted into red lead. 

1st Stage, Dressing. This is conducted in what is called the 
" dressing oven," a kind of reverberatory furnace of which Figs. 13 



94 RED PIGMENTS. 

and 14 show respectively the longitudinal and vertical sections. 
From these drawings it will be seen that it is a low oven, open 
only in front, over which is constructed a hood and chimney 
to carry off the products of combustion, &c., from the furnace. 
The bed usually measures about 11 feet by 12 feet 4 inches, and is 
divided (as shown) into three divisions the central one 
measures 8 feet 4 inches wide, and constitutes the bed or 
hearth of the furnace, while the two side divisions measure 
about 2 feet each, and form the fireplaces of the oven, as a 




Fig. 14. 

rule, they are not fitted with firebars ; the partitions between the 
fireplaces and hearth are formed of firebrick ; the bed of the 
furnace is made to slope from the back to the front, usually the 
back is about 7 inches higher than the front, while it also slopes 
slightly from the side to the centre. In the front of the furnace 
are three doors the two side ones are for feeding the fires, 
while the centre one serves for introducing and extracting the 
material, and for working the charge while in the furnace j it 
is placed a little higher than the two side doors so that a 
draught is generated through the latter and out of the centre 
door; in the top of this door an opening is left so that the 
products of combustion, &c., can pass out and up the chimney. 

This furnace is open to improvement, and an improved form 
is shown in Fig. 15, from which it will be seen that this form of 
furnace has firebars fitted to the fireplaces. 

The operation of dressing is carried out in the following 
manner : 22 cwts. of lead, which is the quantity usually dealt 
with in one charge, are placed in the furnace, which is now 



MANUFACTURE OP RED LEAD. 



95 



raised to a dull red heat, just enough to melt the lead, the 
molten lead being prevented from flowing out of the furnace 
by^ the construction of a dam, formed of pieces of dross or 
" leanings " from previous workings, across the front of the 
hearth ; the melted lead rapidly becomes coated with a layer of 




Fig. 15. 

oxide, the formation of which is hastened by rabbling the lead 
and pushing the oxide as it is formed to the back of the furnace, 
the object being to always have a fresh surface of lead exposed 
to the oxidising action of the air which passes through the 
furnace. The workmen by a peculiar splashing action while 
rabbling expedite this oxidation very much ; at intervals pigs 
of lead are thrown into the furnace. This dressing takes about 
10 to 12 hours, at the end of which time the dam across the 
front of the furnace is broken down, and the uumelted lead 
allowed to run out, while the " dross " or " casing," as it is 
called, is taken out to be worked for the next stage. The 
furnace is now ready for another charge. 

The "dross" or "casing" has a rather bright yellow colour, 
and is coarse in texture ; it consists essentially of the monoxide 
of lead, Pb O, but still contains some unoxidised lead. It is 



96 RED PIGMENTS. 

now ground and levigated with water ; the oxide grinds to the 
form of a fine powder, while the lead is simply flattened out r 
and by sieving can easily be removed ; it is sent back again into 
the furnace, while the ground oxide is washed by a stream of 
water into settling tanks, where it settles out in the form of a 
paste, which is ready for use in the next stage. 

One point of importance in the dressing stage is to see that 
the temperature is carefully regulated, so that, while it is above 
the melting point of the lead and therefore in a molten state, 
yet it is below the melting point of the casing ; as the margin is 
not great, considerable care has to be taken to avoid over- 
stepping the limit. If the casing is allowed to melt it passes 
into litharge and this cannot be converted into red lead. The 
dross or casing is also known as massicot. 

2nd Stage, Colouring. The next operation consists in heating 
the dross obtained in the first stage, either in the same oven or 
in another, which only differs from the dressing oven in a few 
minor details. The colouring oven is heated to a low red heat, 
care being taken to ensure a large supply of air. The operation 
takes about 48 hours, and the mass is frequently rabbled during 
that period; after it has been in about 12 hours a sample is 
taken out and its colour examined ; this sampling is repeated at 
the end of each twelfth hour and near the end of the operation 
more frequently. When the red lead has attained the correct 
colour, the fires are drawn and the furnace allowed to cool 
down; when cold, the red lead is drawn from the oven, ground 
as finely as possible, and sent into the market. 

The change which takes place in the transformation of the 
metallic lead into red lead is shown in the following equations 

1st Stage. Pb + Pb O 

Lead plus oxygen forms lead monoxide. 

2nd Stage. SPbO + O = Pb 3 4 

Lead monoxide plus oxygen forms lead peroxide. 

Theoretically, 100 Ibs. of lead should yield 1 10-36 Ibs. of red 
lead ; practically, about 108 Ibs. of red lead are obtained, which is 
a very near approach to the theoretical amount. The best red 
lead for painters' use is made from pure lead, as the presence of 
impurities in the metalhas a material and injurious influence on 
the colour of the product; iron, in particular, causes the 
colour to be dark. For glass-makers' red lead a pure product is 
absolutely necessary, as an impure lead causes the glass to be 
coloured, not white as it should be. 

Burton's Process. Although the only process at present 



PROPERTIES AND COMPOSITION OF RED LEAD. 97 

worked for the preparation of red lead is the one described 
above, yet in 1862 Burton patented a process for making red 
lead from sulphate of lead, in which 1 equivalent or 1-894 parts 
of lead sulphate are mixed with 1 equivalent or 0*665 part of 
sodium carbonate and 1 equivalent or O143 part of sodium 
nitrate. The mixture is heated to a dull red heat with an excess 
of nitre ; the fused mass is lixiviated with water, whereby the 
red lead formed is separated from the alkaline salts, and this is 
washed and dried. 

PROPERTIES AND COMPOSITION OP RED LEAD. 
Red lead is a heavy, bright red powder of an orange hue, its 
specific gravity being 8-53. Heat turns it to a dark brownish- 
red, but the colour is restored on cooling. Acids act on red 
lead. Nitric acid and glacial acetic acid first dissolve out the 
monoxide, leaving the dark puce oxide ; on further boiling, this 
gradually dissolves and colourless solutions of the nitrate or 
acetate are formed. Hydrochloric acid when heated with red 
lead decomposes it with the evolution of chlorine and the forma- 
tion of the chloride, which settles as the solution cools in the 
form of transparent needles, a very characteristic reaction of lead. 
Sulphuric acid boiled with red lead forms the sulphate, with the 
evolution of oxygen. 

Red lead is a combination of the two oxides of lead, the 
monoxide, Pb 0, and the puce or dioxide, Pb O 2 ; it is generally 
considered that they are present in the proportion of two equiva- 
lents of the first to one of the second, red lead, therefore, having 
the formula Pb 3 O 4 , the percentage composition being 

Lead monoxide, Pb O, . . . 64 '5 
Lead dioxide, Pb 2 . . . 35 '5 

100-0 

There is reason for believing that Pb 3 O 4 does not accurately 
represent the true composition of red lead ; for, although the 
proportions of the two oxides is about that given in the above 
analysis, it is probable that the whole of the monoxide present 
is not combined with the dioxide as red lead, but that some of it 
is in the free condition ; this free oxide is not distinguishable 
from the combined oxides by treatment with acids, but, by 
treating with a 10 to 12 per cent, solution of lead nitrate, it is 
quite possible to extract 16 to 31 per cent, of free oxide, while 
the purified red lead thus obtained contains 25 '4 to 257 per cent, 
of dioxide.* 

* Lowe, Dingl. Polytech. Journ., vol. 271, pp. 472-477. 



98 RED PIGMENTS. 

The formula of red lead would then be Pb 4 O 5 , which is that 
assigned to it by Phillips and other authorities. Percy* gives 
the following analysis of red lead : 

Lead monoxide, Pb O, . . . . 80 '54 per cent. 

Lead dioxide, Pb 2 , . . . . 18 '89 

Ferric oxide, Fe 2 O 3 , . . . . '19 ,, 

Copper and silver, .... trace 

99-62 

which corresponds to the formula 4 Pb O, Pb O, or Pb 5 O 6 . 

Both Pb 3 O 4 and Pb 4 O 5 are known ; the former is much easier 
to prepare than the latter, and the latter can only be made by 
repeated oxidation of the monoxide. A little free monoxide is 
desirable in red lead, as then the colour is not so readily liable 
to spoil by over-oxidation. 

As a pigment red lead is very useful, it mixes very well 
with linseed oil, and takes from 8 to 9 per cent, of it to grind 
into a stiff paste. It exerts a powerful drying action on the oil \ 
hence, paint containing red lead dries very quickly ; on this 
account, also, red lead mixed with linseed oil is largely used as 
a lute and packing for steam pipes and joints of all kinds. It 
possesses good covering and colouring power, and is capable of 
resisting all ordinary atmospheric influences, although it is liable 
to be discoloured by sulphuretted hydrogen as is the case with 
all lead pigments. It may be mixed with nearly all pigments, 
the only exceptions being those containing sulphur, such as 
ultramarine, cadmium yellow, &c. 

ASSAY AND ANALYSIS OF RED LEAD. Red lead 
should be assayed for colour, fineness, and body in the usual way. 
It is rarely adulterated ; but if so, it is usually by the oxide of 
iron reds. The quantity of red lead in such an adulterated 
sample can be ascertained by taking 2 grammes and boiling 
with nitric acid until it is thoroughly decomposed ; the insoluble 
matter can be filtered oft" and its amount ascertained by weighing 
it; to the solution, which is colourless if the red lead be pure, but 
yellow if there is any iron present, a little dilute sulphuric acid 
is added, and a precipitate of sulphate of lead obtained ; this is 
filtered off, washed, dried, and weighed in the usual manner. 
The weight multiplied by 0*955 gives the amount of red lead 
in the sample. The solution from the lead sulphate can be 
tested for iron, &c., by the usual tests. 

* Percy, Metallurgy of Lead. 



OXIDE REDS. 99 



ORANGE LEAD. 

This pigment is identical in composition with red lead, but is 
rather paler in colour and lighter in weight. To make it, white 
lead is placed in a furnace similar to a red lead furnace, 
and heated to a low red heat for from 24 to 48 hours, or until 
the mass has acquired the desired red tint. During this 
operation the white lead loses its carbonic acid and water, while 
it takes up oxygen from the air which passes through the furnace. 

The change is shown in the following equation : 

2 Pb C 3 Pb H 2 2 + = Pb 3 4 + H 2 + 2 C 2 . 

In washing white lead a scum collects on the top of the 
washing waters; this is collected and made into orange lead, and 
gives a brighter and more bulky product than dry white lead. 
Orange lead has a slightly paler colour than red lead, is more 
voluminous and of lower specific gravity, which is about 6*95. 
In its composition and properties it is identical with red lead, 
and it is used for very similar purposes. 



RED OXIDE, INDIAN RED, AND IRON REDS. 

Ferric oxide, Fe 2 O 3 , the red oxide of iron, is the basis of a 
very large number of red pigments which are sold under the 
names of rouge, light red, Indian red, red oxide, Venetian red, 
purple oxide, scarlet red, <fcc., which are all red pigments of 
varying shades of colour. In the hydrated form, ferric oxide 
also forms the colouring principle of the ochres, siennas, and 
umbers. The red oxides are valued very highly as pigments, 
on account of their generally fine colour and their permanence. 

Ferric oxide occurs naturally in a variety of forms in the 
minerals haematite, specular iron ore, limonite, &c., in which it 
is nearly chemically pure. As a rule, they are too dark in 
colour, and too hard to be used as pigments ; but, occasionally, 
deposits of oxide of iron are found of sufficient brilliance to be 
used as a pigment e.g., the Warton oxide named below. Indian 
red was originally found native, but is now mostly of artificial 
manufacture. Such substances as red ochre, yellow ochre, red 
raddle, umber, &c., owe their colour to oxide of iron, but they 
contain many other bodies. 

These pigments are prepared both from natural and artificial 
sources. Before entering into the details of their preparation a 



100 RED PIGMENTS. 

few words about the oxides of iron and their properties will be 
of use in understanding the subject. 

There are three principal oxides of iron, viz. : 

Ferrous oxide, the green or protoxide, Fe O. 

Ferric oxide, the red or peroxide of iron, Fe 2 O 3 . 

Triferric tetr oxide, the black or magnetic oxide, Fe 3 O 4 . 

Ferrous oxide (iron monoxide, protoxide of iron, as also, from 
its colour and the colour of its salts, the green oxide), Fe 0, is 
not known in the free condition. When a solution of caustic 
soda or of caustic potash is added to a solution of a ferrous salt a 
green precipitate of the ferrous hydroxide, Fe H 2 , is obtained; 
this, when exposed to the air, rapidly oxidises and passes into 
the ferric compound, the change being accompanied by a change 
of colour from green to red ; even when kept under water this 
change goes on, although slowly ; on heating it changes to the 
ferric oxide. Except in a few rare minerals ferrous oxide is not 
known to exist naturally. Ferrous hydroxide is soluble in all 
acids, with the formation of the corresponding ferrous salt ; these 
salts are only stable when in the dry condition, when dissolved 
in water they gradually change into the ferric salts, and usually 
a small quantity of ferric oxide is formed at the same time. 

Ferric oxide (iron trioxide, sesquioxide of iron, peroxide of 
iron), Fe 2 O 3 , is the most important of the three oxides of iron. 
It is found in great abundance in nature in a great variety of 
forms, some of which have been noticed above. It generally 
occurs in a nearly pure state, and is then used as an ore of 
iron ; at other times it is found mixed with earthy and other 
substances in variable quantities, such bodies are used for 
several purposes; some of them, which have been named above, 
as pigments. It is usually of a dark red colour, although some 
forms are nearly black. When it occurs in a hydrated condition, 
as in limonite, or as it is in yellow ochre, it has a brownish- 
yellow or brown colour. Ferric oxide is soluble in acids, easily 
in strong hydrochloric acid, or a mixture of nitric and hydro- 
chloric acids ; it is not so readily soluble in sulphuric or nitric 
acids ; with these acids it forms the ferric salts ; but the degree 
of solubility is very variable, some forms being almost insoluble, 
while others are freely soluble. It is perfectly stable, and can 
be exposed to the air for any length of time without change. 
When a solution of caustic potash or soda or ammonia is added 
to a solution of a ferric salt a brownish-red, nocculent precipitate 
of the ferric hydroxide, Fe 2 H 6 O 6 , is obtained ; on being heated 
to a low red heat this loses its water and passes into the 
oxide, Fe 2 O 3 . ' 



MANUFACTURE OF OXIDE REDS. 101 

Triferric tetr oxide (magnetic oxide of iron), Fe 3 O 4 , is found 
naturally in large quantities as the mineral magnetite, the 
natural loadstone; this is used as an ore of iron; it is not 
readily soluble in acids, and its solution contains both ferrous 
and ferric salts. 

A fourth oxide, ferric anhydride, Fe 3 , is known in the 
form of a compound salt of potassium, corresponding to the 
chromate of potassium ; it is not, at present, of practical 
importance. 

MANUFACTURE OF OXIDE REDS. These are ob- 
tained, as stated above, from (a) natural and (6) artificial sources. 

(a) Preparation from Natural Oxide of Iron. The preparation 
of oxide reds from the iron minerals is comparatively simple. 
The mineral is first ground in an edge-runner grinding mill so as 
to reduce the large masses to small pieces ; then these are sent 
several times through a roller mill, until they are fine enough, or 
they may be put through a horizontal stone mill fitted with a 
hopper for levigating and the ground pigment levigated to free it 
as much as possible from grit ; after this, the levigated material 
requires to be dried before it can be sent out. Descriptions of 
grinding mills, levigating and drying processes will be found in 
subsequent chapters. 

As a rule, the natural oxide reds are fairly pure products ; 
some analyses will be found on p. 109. They are generally dark, 
and are mostly sold as Indian red and red oxide. They are 
excellent and very permanent pigments. 

(b) Preparation from Artificial Sources. Oxide of iron reds 
are prepared from a variety of materials, some of which are 
obtained as waste products in other branches of manufacture, 
while others are used directly for making them. The manu- 
facturer is able, within certain limits, to prepare any shade of 
red which is desired ; the darker shades are much more easily 
prepared than the paler shades ; the former may be almost 
chemically pure, but the paler shades can only be made from 
such bodies as the ochres, which only contain a comparatively 
small quantity of iron. 

The artificial oxide reds are sold under a great variety of 
names. In some cases these originally showed the origin of the 
pigment when it was first introduced, e.g., Indian red and Vene- 
tian red; while others, such as purple oxide, light red, and scarlet 
red are indicative of the shade of colour. The names, as now 
used, are usually indicative of the shade of the red, thus rouge, 
colcothar, scarlet red, and Turkey red are bright reds ; Venetian 
red is a pale red ; light red is rather darker, but is usually paler 



102 RED PIGMENTS. 

than those above enumerated ; Indian red and red oxide are dark 
reds ; while light and dark purple oxides are of a dull violet hue. 
The processes which are used in the preparation of oxide reds 
may be grouped under two heads : 

1st. Dry Processes. 
2nd. Wet Processes. 

In the first group the reds are made by heating an iron 
compound. 

In the second, a precipitate of iron is thrown down from solu- 
tion, and is then usually heated to convert it into the red. 

1st. DRY PROCESSES. (a) From Copperas. The prin- 
cipal material used in the preparation of these reds by the dry 
processes is copperas, ferrous sulphate. This iron salt is made in 
very large quantities by exposing iron pyrites to the oxidising 
action of the air, neutralising the acid solution of sulphate thus 
obtained by means of scrap iron and then crystallising out the 
ferrous sulphate so formed. Copperas, ferrous sulphate, forms 
large, pale green crystals, which have the formula Fe 8 O 4 , 7 H 2 O. 
On exposure to the air they are liable to oxidise and to form a 
brown crust of the ferric oxide. Heated at about 120 C. they 
lose 6 of the 7 molecules of water of crystallisation they contain, 
falling to a white powder in so doing ; at a higher temperature 
they lose the seventh molecule of water. Copperas dried by heat 
is much used as a drier in varnish making. Partially-dried 
copperas, when heated to a white heat, or even to a lower tem- 
perature, is decomposed; it loses its sulphur trioxide, while a 
residue of ferric oxide remains ; some of the sulphur trioxide is, 
however, decomposed into sulphur dioxide, the oxygen it losea 
going to the iron. The change is expressed in the equation 

2FeSO 4 = Fe 2 3 + S 2 -H S 3 

When carried out on a large scale, the copperas is rarely 
thoroughly dried, so that the water it contains passes off with 
the sulphur oxides, and forms with them a brownish fuming 
liquid known as Nordhausen or fuming sulphuric acid ; in fact, 
this was the first process by which sulphuric acid was made, but 
it is now displaced by the English method ; the iron oxide which 
is left behind is sold for various purposes under the name of 
rouge or colcothar. 

The process may be carried on with a view of obtaining the 
acid as the principal product and the oxide as a bye-product, or 



MANUFACTURE OF IRON REDS. 103 

the oxide may be the principal product and the acid the bye- 
product, as is the case when copperas is calcined. 

When the process is carried on with a view to obtain the acid 
which is evolved, the operation is generally done in cast-iron 
stills which are fitted with suitable condensing arrangements. 
The stills are heated to a red heat ; after the evolution of the 
acid vapours has ceased, the still is allowed to cool, and the resi- 
dual oxide ground with water to wash it and grind it as fine as 
possible ; then it is dried and sent out as rouge or colcothar or 
scarlet oxide, &c. The oxide must be washed to free it from all 
traces of the acid, which would reduce its value as a pigment if 
left in. 

The usual plan is to calcine the dried copperas, either with or 
without previous drying, in a form of muffle furnace, such as is 
shown in Fig. 16, which may be arranged with a view to collecting 
the acid gases evolved or to allow them to pass away without 
being collected, as is perhaps the most common plan ; but in 
view of the value of sulphuric acid, it is worth while to collect 
and manufacture the acid gases into the acid, especially if the 
oxide manufacture be carried on so far from an acid works that 
the cost of the sulphuric acid required for other operations in 
the colour works becomes an important item. 

By regulating the temperature at which the calcination is 
carried on, and also its duration, the shade of the resulting oxide 
may be modified to suit the requirements of the colour maker ; 
thus if carried on at a dull red heat a bright shade of red is 
obtained ; if at a red heat, a darker red ; while at a white heat, 
the oxide will take a dark violet shade. The same effects may 
be produced by prolonging the calcination at a lower temperature, 
but the results are usually not so good. During the calcination it 
is necessary to take out samples from time to time to see how the 
operation is proceeding. When the mass has acquired the right 
shade the operation is stopped by drawing the furnace fire, and 
the furnace and its contents are allowed to cool down ; or the 
oxide may be withdrawn while still hot, leaving the furnace in a 
condition to take another charge without loss of time and with 
some saving of fuel. 

Although it is not necessary to add anything to the copperas 
in order to produce the oxide, some makers add a small quantity 
of salt, about 2 to 3 Ibs. per cwt. of copperas ; other makers add 
sulphur; but there cannot be much advantage in adding these 
bodies, especially the latter. The salt may act by increasing the 
bulk of the resulting oxide and thus tending to brighten the 
shade somewhat. When these bodies are added it is necessary 



OFTHF. 
vr T TT -r-i -r-^ _ 



104 



RED PUaiENTS. 



that the oxide be very carefully washed before it is sent out, as 
if any salt, &c., be left in it will interfere very materially with 
the use of the oxide as a pigment. 

Messrs. Leech & Neal have patented the use of a kiln con- 
structed very similar to a brick kiln, the gases evolved from 
which are passed into a lead chamber, where the sulphuric acid 
they contain is condensed, while the sulphurous acid passes into 
a column with water and scrap iron, dissolving the latter and 
forming ferrous sulphate, which is used for another batch of 
oxide. 

Lavender describes in a patent a method of preparing oxide 
from the waste liquors of iron galvanisers which, if they have 
used sulphuric acid for pickling the iron, contain sulphate of that 
metal, by passing superheated steam through them, thus driving 
off the water and excess of acid and drying the sulphate, which 




Fig. 16. 

is then calcined in kilns so built that the gases evolved can be 
condensed in lead chambers. 

In another patent Madge has described a furnace and process 
for preparing oxide from copperas. The furnace is shown in 
Fig. 16, and is of the muffle form, with arrangements for collect- 



MANUFACTURE OF IRON REDS. 105 

ing and condensing the sulphur gases evolved. The copperas is 
first dried on iron plates heated by the waste heat from the 
muffles ; then, after drying, it is transferred to the muffles and 
further calcined to oxide, which is then finished as usual. 

The oxide yielded by the above processes is fairly pure and 
mostly of a dark red shade, which, as has been mentioned above, 
may be varied at will to some extent, but the pale shades cannot 
be obtained in this way. 

(b) From Ochres. By calcining ochres, an operation which is 
carried on on a large scale, reds are obtained which vary in tone 
very much and to a greater extent than is possible in the case of 
the oxides prepared as above described from copperas. The 
process was patented as early as 1794 by Atkinson. The furnace 
for calcining ochres varies in form at different works ; in some 
places it is simply a stone slab or iron plate placed over a fire ; 
in others some form of muffle furnace is used ; indeed, almost any 
form of furnace may be used. 

The ochres should be ground and levigated before they are 
calcined, as the red obtained from ground ochres is better than 
that obtained from unground ochres. For Venetian reds about 
8 hours at a low red heat is required ; light reds require about 
10 hours ; Indian reds about 12 hours ; while purple reds require 
16 hours. With higher temperatures less time is required. 
During the progress of the calcining test-samples should be taken 
out from time to time to see how it is proceeding, and when the 
red has assumed the required shade it must be withdrawn from 
the furnace. As soon as it is cold it is ready for the market, 
provided it has been ground and levigated before heating, other- 
wise it must be ground and levigated before it is fit to use as a 
pigment. 

The oxide of iron exists in the ochre in a hydrated condition 
(see Ochres, p. 133), and this has a brownish tone. On calcining, 
this water of hydration is driven off, and the oxide, becoming 
converted to the anhydrous condition, turns red. 

The iron reds made from ochres are necessarily impure, as the 
oxide of iron is associated with silica, alumina, lime, &c., their 
composition varying with that of the ochres from which they are 
made. Some analyses of these reds will be found on p. 109. 

2nd. WET PROCESSES. A very large proportion of the 
oxide reds now sold are prepared by a combined precipitation 
and calcining process which can be grouped briefly as "wet 
processes." 

In a large number of metallurgical operations liquors con- 
taining iron are produced in large quantities ; formerly these 



106 RED PIGMENTS. 

were thrown away as useless, but now the iron they contain is 
recovered in the form of red oxide, which is used as a pigment. 
Processes for extracting the iron from these liquors have been the 
subject of numerous patents, some of which have been worked on 
the large scale, while others have not gone beyond the experi- 
mental scale ; some are for the actual process, while others are 
for the plant which is required for the purpose, and some are for 
both process and plant. It is not necessary to describe these in 
detail, but an outline of the principal processes will be given. 

In galvanising or tinning iron goods to preserve them from 
the rusting action of the atmosphere the iron goods are first 
dipped into dilute sulphuric or hydrochloric acid to clean them 
free from any oxide before placing them in the zinc or tin bath ; 
necessarily the acid liquor will become charged with iron to a 
greater or less extent; formerly this acid liquor was thrown 
away, but galvanisers have been compelled to adopt some plan 
of treating it as the various local authorities interested objected 
to its discharge into drains and watercourses. The iron is 
recovered by precipitating it in the form of oxide by the addition 
of an alkaline solution. For this purpose soda crystals, caustic 
soda, ammonia, ammonia carbonate, or lime may be used ; the 
latter is the cheapest, but its use requires some care, as, if used 
in excess (which may easily happen), the excess will find its way 
into the drains or watercourses, where its presence is rather 
objectionable. By any of these bodies the iron is practically 
precipitated in the form of oxide ; as a bye-product there will be 
obtained some alkaline compound, the nature of which will 
depend upon the nature of the alkali used and of the acid 
employed in the galvanising operation. If soda is used, either 
chloride or sulphate of soda will be formed in the liquors; these 
may be thrown away, as the quantity obtainable is not worth 
recovering ; if ammonia is used (and for this purpose the crude 
ammonia liquor of gas works may be used), either ammonia 
chloride or sulphate will be formed, from which ammonia may 
be recovered. When lime is used, either chloride or sulphate 
is obtained ; in the former case it will pass away in solution, 
while in the latter it will pass into the precipitate and will 
modify the shade of the resulting pigment. 

The general method of procedure is as follows : The liquors 
are run into tanks and the acidity of them is destroyed by adding 
scrap iron; then the alkaline liquor is added in sufficient 
quantity to precipitate the whole of the iron ; the precipitate is 
collected on filters, dried by any convenient means, and then 
calcined in a furnace. If a Venetian red is wanted, after pre- 



PROPERTIES OF RED OXIDES. 107 

cipitating, a quantity of sulphate of lime is added, so that there 
shall be about three or four parts of sulphate to one of oxide ; 
this addition is generally only necessary when the iron liquors 
contain hydrochloric acid and not sulphuric acid. 

The use, during recent years, of large quantities of Spanish 
cupreous pyrites in the manufacture of sulphuric acid, and the 
extended use of wet processes for the extraction of the copper 
from the residual oxide left behind after the sulphur has been 
burnt off, has placed at the disposal of the colour maker new 
and cheap sources of waste iron liquors. In the manufacture 
of sulphuric acid the pyrites is burnt in kilns, the sulphurous 
acid gas evolved is sent into lead chambers to be converted 
into sulphuric acid, while the burnt ore is heated in a suitable 
furnace with salt, which converts all the copper and some of the 
iron into chlorides. The furnaced mass is now dissolved in 
water, scrap iron is thrown into the liquors thus obtained to 
precipitate the copper, while the iron passes into solution ; the 
iron liquors thus obtained are a waste product so far as the 
copper extractor is concerned, but the colour maker can use 
them for making red oxide. 

The iron in these liquors generally exists in the form of 
chloride and sulphate ; from them the oxide is precipitated by 
adding lime, after which the precipitate is collected, dried, and 
calcined in the usual way. The oxides so obtained nearly always 
contain sulphate of lime, which is precipitated along with the 
iron on adding the lime to the iron liquors; this sulphate of 
lime tends to make the oxide of a lighter shade than it would 
otherwise be. During the last few years many patents have 
been taken out for treating these waste iron liquors for oxide 
making, in the specifications of which full details will be found 
of the processes adopted. 

- PROPERTIES OF RED OXIDES. Red oxide pigments 
form red powders of various shades, from a pale red to a dark 
violet, the specific gravity of which varies from 2'6 to 3*1 j they 
are quite insoluble in water, and more or less insoluble in acids. 
If the oxide has been made at a low temperature, as for instance 
rouge, it will dissolve in strong hydrochloric acid ; if it has 
been made at a higher temperature, Indian red for example, it 
is not readily soluble in hydrochloric acid, and requires a 
mixture of that acid and nitric acid to effect its solution ; while 
the purple oxides which have been prepared by calcining at a 
very high temperature are very insoluble bodies, and require to 
be treated with a mixture of sulphuric, hydrochloric, and nitric 
acids before they will dissolve. This point regarding the 



108 RED PIGMENTS. 

solubility of oxide reds should be borne in mind when making 
an analysis of these bodies. Of course it is only the pure 
oxides which are soluble, those made from ochres always yield 
an insoluble residue of some kind. 

As a pigment, red oxides are perfectly permanent under all 
conditions, and are among the most permanent pigments a 
painter can use. They mix perfectly with all pigments without 
either affecting them in any way or being affected by them. 
They do not mix as well with oil as red lead, take about 
10 per cent, of oil to grind into the usual stiff paste, and do 
not act as driers. 

ANALYSIS AND ASSAY OP RED OXIDES. From 
what has been said above as to the methods of preparation of 
red oxides it will have been gathered that these bodies are of 
very variable composition ; some are nearly pure oxide, others 
contain calcium sulphate, while others are much more complex, 
containing silica, alumina, calcium, &c. In technical work it is 
rare that the absolute purity of a body is an essential feature, 
and this applies, as a rule, to pigments particularly ; with them 
purity is not essential, provided the impurity is of such a 
character as will not affect the use of the pigment as a pigment ; 
for example, sulphate of lime may be present, but not caustic 
lime, as the former will not, beyond making the shade lighter, 
affect the pigment injuriously, while the latter will. It is 
rarely, therefore, that an analysis of a pigment is required, and 
this is particularly the case with red oxides. 

When an analysis is required of an oxide red the following 
scheme may be adopted : 

Water. Weigh 2 grammes into a watch-glass, and heat in 
a hot oven to a temperature of about 120C. until there is no 
further loss of weight. This gives the hygroscopic water. For 
the combined water weigh 2 grammes into a crucible, and 
heat to a red heat over the Bunsen burner for about one hour, 
then, after allowing to cool, weigh ; the loss of weight is the 
hygroscopic plus the combined water. 

Next weigh into a beaker 5 grammes, and heat with a 
mixture of hydrochloric and nitric acids until the red is com- 
pletely decomposed ; then evaporate the solution to dry ness, treat 
the dry mass with a little dilute hydrochloric acid, filter off into 
a 500 cc. flask, wash well with water, and fill the flask up to 
the mark. This solution is used for the various tests given 
below. The residue on the filter is the silica, barytes, <fec., 
which can be dried and weighed. 

For the iron, alumina, &c., take 200 cc. of the above solution, 



ANALYSES OF RED OXIDE OF IRON PIGMENTS. 



109- 



3^88 .2 .8 
'"'g^ : 8' : ^ 



i ! ^ 



* "-i 

o .o 



.oca 



_l O i 1 r I O 

C5 . CC ^ - 



oo p^ 2 . . pco 

CO CO CO CO " " 

^ CM CM 



SCN 00 -* O 
. -*^9-T : : : 

OS ^H 05 O 



s 



OS O O O 

1O - - -CD - !> O 



t^* CO 00 CO 

* 



* 



8 



8 



S 






ff 



fl * 

fc 1 

W I a> 

I 8 . 1 



- 



.s 



*" 



c3 S O 

Jz; g ^ ^ pm pi 



110 RED PIGMENTS. 

add carbonate of soda until the solution is neutral, then ammonium 
acetate ; boil, filter, wash and dry, and weigh the precipitate this 
consists of oxide of iron, alumina, and, in some rare cases, phos- 
phoric acid, but this may be neglected, as a rule, in iron reds. 
The iron may be estimated in another portion of the original 
solution by a volumetric test and the amount of alumina 
calculated from the two results. The nitrate from the precipitate 
is mixed with a small quantity of ammonium sulphide to pre- 
cipitate any manganese, this precipitate being collected, dried, 
and weighed. To the filtrate is added ammonium oxalate to 
precipitate the calcium, which is filtered off, dried, and weighed. 
To the filtrate from this, sodium phosphate is added to precipitate 
the magnesium, if present. 

For the sulphate which is present 100 cc. of the original 
solution are taken and some barium chloride is added ; the pre- 
cipitate of barium sulphate is filtered off, dried, and weighed. 

For fuller details as to the method of carrying out this scheme, 
works on quantitative analysis, such as that of Professor Sexton, 
published by Griffin & Co., should be consulted. 

The analyses on p. 109, which, with the exception of Nos. 3 
and 7, have been made by the author, will show the composition 
of the iron reds in common use as pigments. 

A pigment must possess two properties, good colouring power 
and body. As the iron reds are so variable in their composition 
it follows that in these two particulars they will vary also ; for 
these two properties they should be assayed, the methods of 
doing which will be found detailed in the chapter on assaying 
pigments. 

ANTIMONY VERMILION. 

This pigment, also known as antimony orange, is very largely 
used for colouring india-rubber; for other purposes it is not used 
as extensively as it might be. 

It is the sulphide of the metal antimony and has the formula 
Sb 2 S 3 . This body occurs naturally as the mineral stibnite or 
antimony glance of a lustrous black colour ; when ground up it 
is known as black antimony, and is used for various purposes in 
the arts, one being as a source for the manufacture of antimony 
vermilion. 

PREPARATION OP ANTIMONY VERMILION. 
Murdoch, in 1847, patented a process for the preparation of anti- 
mony vermilion and a similar process was the subject of a subse- 
quent patent taken out by Clark. The pigment can be made in 



SCARLET ANTIMONY. 



Ill 



two modifications, orange and red ; the former by precipitation 
with sulphuretted hydrogen, the latter by other agents. 

(a) Orange Antimony. Murdoch dissolves the black anti- 
mony in hydrochloric acid; during the operation some sulphu- 
retted hydrogen is evolved and may be used for precipitating 
another solution previously made ; so as to form a solution of 
antimony chloride, which is concentrated till it has a strength of 
19 Tw. ; through it is then passed a current of sulphuretted 
hydrogen gas which precipitates the sulphide of antimony as an 
orange powder, which, after being well washed and dried, is ready 
for use. 

Fig. 17 shows a convenient form of apparatus for precipitating 
antimony vermilion. A is a vessel made of wood, lined with 
lead, and fitted with a lid which, while being removable to admit 
of the sulphide of iron used for the preparation of the gas, can 




A C D D D 

Fig. 17. Apparatus for making antimony vermilion. 

yet, by means of india-rubber joints, be made gastight ; B is a 
funnel for the admission of acid ; C is a two-necked bottle filled 
with water to wash the gas as it comes over ; D,D,D, are three- 
necked bottles (whose construction is shown in the drawing), con- 
taining the solution of antimony through which the gas is passed. 
The most convenient method of preparing the gas is by the action 
of dilute sulphuric acid on sulphide of iron. 

By varying the strength of the antimony solution the shade of 
the resulting pigment can be varied to some extent ; thus, a solu- 
tion of 19 Tw. gives an orange-red; one of 40 Tw., a reddish- 
orange ; while one of 52 Tw. gives an orange colour. When 
strong solutions are used the precipitate is, however, liable to 
contain free sulphur, which is sometimes objectionable as it may 
cause decolourisation of the pigment. 

(6) Scarlet Antimony. 1. Mathieu Plessy Process. This con- 
sists in precipitating a solution of chloride of antimony with a 
solution of sodium thiosulphate (hyposulphite of soda) under 



112 KED PIGMENTS. 

certain conditions. A solution of chloride of antimony of 40 
Tw. is prepared. The ordinary commercial chloride is a liquid of 
about 1-26 (52 TV.) specific gravity; if this is diluted with 
water in the proportion of 5 vols. of the chloride to 2 vols. of 
water, a solution of about 40 TV. will be obtained. A solution 
of sodium thiosulphate of 40 Tw. is also prepared ; this will take 
about 14 ozs. weight of the salt to 25 ozs. measure of water. 

7 1 gallons of the thiosulphate solution are taken, and into them 
are poured 3 gallons of the antimony solution; a black precipitate 
forms at first, but this disappears rapidly. The liquid mixture 
is now gently heated ; when the temperature reaches about 78 
to 90 F. a yellow precipitate begins to form ; as the temperature 
increases the colour changes, passing through various shades of 
orange till at about 130 to 140 F. it acquires a scarlet-red 
colour ; the operation is now stopped and the mass allowed to 
cool down ; when cold, the clear supernatant liquor is poured off, 
water containing a trace of hydrochloric acid is poured on to the 
precipitate, which is stirred up ; after which the mass is allowed 
to stand to settle, the top liquor poured off, and the colour 
washed with water two or three times ; it is now dried at a low 
temperature (about 140 F.), and when dry is ready for use. 

During the process of precipitation a very considerable evolu- 
tion of sulphur dioxide gas takes place ; therefore, the operation 
should be carried on in a place and under conditions where the 
gas cannot cause inconvenience. 

2. Wagner's Process. 4 Ibs. of tartar emetic and 3 Ibs. of 
tartaric acid are dissolved in 18 Ibs. of water, and the solution 
heated to 140 F., a solution of sodium thiosulphate of 40 
Tw. added thereto, and the mixture heated to 180 F. The 
red is gradually precipitated, and when fully formed is washed 
with water and dried. 

In drying antimony vermilion it is important that the 
temperature be kept low, and not be allowed to rise above 
160 F. 

The colour of the scarlet antimony powder is rather dull, but 
it becomes bright when mixed with oil or water. 

PROPERTIES OF ANTIMONY VERMILION. Anti- 
mony orange is a light, bulky, orange-coloured powder, while 
antimony vermilion is a scarlet powder rather heavier than 
the orange variety. Both pigments have practically the same 
properties; they are not attacked by dilute acids, but strong 
nitric acid gradually decomposes them, with the formation 
of white antimonic oxide and sulphuric acid. Strong hydro- 
chloric acid has little action in the cold, but when boiling 



COMPOSITION OF ANTIMONY VERMILION. 113 

gradually dissolves them with the formation of chloride of anti- 
mony and evolution of sulphuretted hydrogen. Caustic soda and 
potash when boiled dissolves the colour, which is re-precipitated 
as an orange precipitate on the addition of an acid ; ammonia 
has little action ; lime has a similar action to soda. 

Both the orange and red forms are rather dull in the pulveru- 
lent state, but when mixed with oil or water they become bright; 
being opaque they have a good body or covering power, and mix 
well with oil, but cannot be used with alkaline vehicles, like 
lime or silicate of soda, which have a decolourising action. They 
are unalterable by air or light, or by deleterious atmospheric 
agents. 

They can be mixed with all those pigments which are 
unaffected by sulphur. 

COMPOSITION OP ANTIMONY VERMILION. 
Pure sulphide of antimony, Sb. 2 S 3 , has the following percentage 
composition : 

Antimony, 71'42 

Sulphur, 28-28 

100-00 

The antimony pigments are liable to contain free sulphur, 
especially those which are made with sulphuretted hydrogen. 
The following is an analysis of an orange antimony : 

Moisture, 2-200 per cent. 

Sulphur, 40-557 

Antimony, 56 '990 

99-747 

from which it will be seen that the sulphur is greatly in excess 

of that required by theory, so that some of it must be in the 

free condition. 

The red antimony vermilions approach more closely to the 

theoretical composition, as will be evident from the following 

analyses : 

a. b. 

Water, ...... I'l 4 -227 

Sulphur, 26-7 27'103 

Antimony, 72'2 68'670 

100-0 100-000 

the latter analysis by the author is of an English made sample, 
the other analyses are taken from a foreign work on pigment- 
making. 

8 



114 RED PIGMENTS. 

The presence of free sulphur in these pigments is likely to 
be a cause of want of permanence when used as a pigment. 

ANALYSIS AND ASSAY. For practical purposes it is not 
necessary to make a complete analysis of antimony vermilions. 
They are liable to adulteration by red lead, oxide of iroD, or 
chrome orange. Red lead would be shown by the colour dark- 
ening when treated with hydrochloric acid, and, after solution, 
by crystals of chloride of lead separating out on cooling, and the 
application of the usual tests for lead. Oxide of iron can be 
distinguished by the solution in hydrochloric acid having a 
yellow colour and giving the characteristic tests for iron ; 
chrome orange can be detected by the colour of the acid solution 
being green and giving the tests for lead and chromium. 

Antimony vermilions should be assayed for colour, both in 
the form of powder and when mixed with oil, and for covering 
power by the usual methods. 

BRILLIANT SCARLET is the name given to the iodide of 
mercury, Hg I 2 , prepared by carefully precipitating a solution of 
mercuric chloride with a solution of potassium iodide ; it has a 
brilliant scarlet colour, but is very fugitive. It is rarely used 
as a pigment. 

DERBY RED is the basic chromate of lead ; its preparation 
and properties will be found on p. 116, et seq. 

The CHROMATE OF MERCURY has been used as a red 
pigment and is prepared by precipitating a solution of mercuric 
chloride with potassium chromate ; its cost and want of per- 
manence has caused it to become obsolete as a pigment. 

The CHROMATE OF SILVER has been proposed as a pig- 
ment ; it has a dark red colour and is prepared by precipitating a 
solution of silver nitrate with a solution of potassium chromate. 
It is costly and fugitive. 

The CHROMATE OF COPPER, a dark red coloured body 
prepared by precipitating solutions of copper with potassium 
chromate, has also been suggested, and probably used on a small 
scale, as a pigment ; but it is fugitive, and, therefore, cannot be 
recommended for this purpose. 

MAGNESIA PINK, prepared by calcining a mixture of 
magnesia and cobalt nitrate, has been little used ; it has but a 
pale colour and little body, although probably permanent. 



115 



CHAPTER IV. 
YELLOW AND ORANGE PIGMENTS. 

THERE is a fairly large list of yellow and orange pigments 
derived from the vegetable, animal, and mineral kingdoms, 
the most important being the chromes and ochres; the others are 
only used in small quantities. 

THE CHROMES. 

The chromes, as they are generally called, are a very important 
group of pigments varying in colour from a pale yellow through 
deep yellow, orange to bright red, and sold under a variety of 
names primrose-chrome, lemon-chrome, chrome-yellow, orange- 
chrome, scarlet -chrome, chrome-red, Derby-red, American- 
vermilion, &c. The base of all these chrome pigments is the 
chromate of lead, Pb Or O 4 , and its basic derivative Pb O, PbCr O 4 . 

Chromate of lead is capable of existing in the form of acid, 
normal and basic modifications ; to the colour maker, only the 
last two are of any interest. 

The normal chromate of lead is a deep-yellow coloured body 
having the composition 

Lead, 63 '99 per cent. 

Chromium, 16 '23 

Oxygen, 1978 

100-00 

or 

Lead oxide, Pb O, .... 68 '93 per cent. 
Chromium oxide, Cr Oa, . . . 31*07 ,, 

100-00 

It is obtained as a bright yellow precipitate by adding a 
solution of bichromate of potash to one of acetate of lead ; the 
reaction is shown in the following equation : 

Pb2C 2 H 8 2 + K 2 Cr 2 7 + H 2 O = 2PbCr 4 + 2KC 2 H 3 2 
+ 2 H C 2 H 3 O 2 . 



116 YELLOW AND ORANGE PIGMENTS. 

Nitrate of lead may be used instead of the acetate, when the 
reaction becomes 

2Pb2N0 3 + K 2 Cr 2 O 7 + H 2 = 2PbCr0 4 + 2KN0 3 + 2HN0 3 . 

From the above equations the equivalent quantities of the two 
compounds can be calculated, and are as follows : 650 parts of 
lead acetate or 662 parts of lead nitrate are equal to 295 parts 
of potassium bichromate, or 100 parts of lead nitrate require 
44*5 parts of potassium bichromate, while 100 parts of lead 
acetate require 38*9 parts of potassium bichromate to precipitate 
them. 

Lead chromate is insoluble in acetic acid and water, but 
soluble in moderately concentrated nitric or hydrochloric acids. 
When treated with a large excess of caustic potash or soda 
it dissolves, but when heated with a small quantity of the alkalies 
it is converted into the basic chromate, Pb. 2 Or O 5 . Heated alone 
it first turns reddish-brown, and, finally, becomes greenish-grey 
with evolution of oxygen, while a mixture of the oxides of lead 
and chromium are left behind. 

It has a specific gravity of 5-653. 

The formation of the basic chromate from the normal chromate 
is shown in the equation 

2PbO0 4 + 2NaOH = Na 2 CrO 4 + 2Pb. PbOCr0 4 . 

It is a scarlet-red powder of somewhat crystalline structure, 
with a specific gravity of 6-266 ; by friction it loses its crystalline 
form and changes colour, becoming orange ; in other properties 
it resembles the normal chromate. It has the following com- 
position : 

Lead, 75 75 per cent. 

Chromium, ..... 9'61 ,, 

Oxygen, 14'64 

100-00 
or 

Lead oxide, Pb 0, . . . . 81 -61 per cent. 
Chromium trioxide, Cr 3 , . . 18'39 ,, 



100-00 



Both the normal and basic chromates when boiled with strong 
sulphuric acid are decomposed, lead sulphate and chromium sul- 
phate being formed and oxygen evolved. Strong hydrochloric 



MANUFACTURE OF LEAD CHROMES. 117 

acid on boiling dissolves them, forming a green solution of lead 
and chromium chlorides, from which, on cooling, the lead chloride 
separates out, and the chlorine is evolved. When boiled with 
solutions of the alkaline carbonates, the chromates are decom- 
posed, white carbonate of lead being formed and a solution of the 
alkaline chromate obtained. When boiled with solutions of 
caustic soda or potash the lead chromates dissolve ; on adding 
acetic acid to this solution a yellow precipitate of the normal 
chromate is obtained. 

MANUFACTURE OF LEAD CHROMES. In commerce 
the pigments having chromate of lead as the base are met with 
in a very great variety of shades from a very pale primrose-yellow 
to a deep red ; as a rule, the deep shades are almost chemically 
pure, but the pale shades are obtained by mixing the pure 
chromate with the requisite quantity of a white base, such as 
sulphate of lead, barytes, gypsum, &c. ; what are called "pure 
chromes " in the trade contain sulphate of lead, while the com- 
mon chromes contain this body with barytes or some other white 
base in addition. 

The preparation of these pigments may be grouped under three 
heads : 

1st. Preparation of chrome-yellows. 

2nd. Preparation of chrome-oranges and scarlets. 

3rd. Preparation of chrome-reds. 

The same plant may be used in the preparation of all these 
chromes. 

The best form is that shown in Fig. 18 which, in its simplest 
form, consists of three tubs made of hard wood ; one of these, 
the largest, C, is placed on the floor, while the other two are 
placed above it on a small platform, P. In these two tubs 
(D D of the Fig.) are dissolved the salts used, the bichromate 
of potash and other precipitants in one, the lead salt in the 
other ; care being taken always to use the same tub for the 
same material. These tubs may measure 3 ft. 6 in. by 2 ft. 
6 in. ; they will hold about 107 gallons each. The larger tub, 
C, may measure about 4 ft. in diameter by 3 ft. high, and 
will hold about 176 gallons. In this tub the colour is formed, 
and may, therefore, be called the precipitating or colour tub. 
The contents of I) D may be heated by an arrangement of 
steam pipes, S S, and the colour tub should also be provided 
with a steam pipe, so that its contents can be heated up if 
required. At the bottom of D D are plugs, on opening which 



118 



YELLOW AND ORANGE PIGMENTS. 



the contents can be run into the colour tub, C. For the purpose 
of running off the liquor from the precipitated colour in C, the 
latter is provided with a number of holes, H, H, H, H, at various 
heights in its side fitted with plugs, any one of which may be 
drawn out to permit the water or liquor to run away. 

In a plant of this kind about 2 cwt. of chrome may be made at 
one batch. It is advisable to provide several such sets of plant, 
which may be arranged as shown in the drawing, in which case 
one of the solution tubs, D, may be used along with two colour 
tubs, C 0. 

1st. PREPARATION OP C HBO ME -YELLOWS. The 
number of recipes which are in use for preparing chrome-yellows 




Fig. 18. Plant for preparing chrome -yellow. 

is legion. Most colour-makers will produce from 6 to 10 or 
12 shades, and it is quite probable that the recipes in use in 
different works for producing what is nominally the same shade 



PREPARATION OF CHROME-YELLOWS. 119 

differ very much. Under these circumstances it will be best to 
describe the production of a few of the more prominent shades, 
and point out how, by various modifications in the recipes, others 
can be obtained. 

All the chrome-yellows are prepared by a comparatively simple 
and easily-conducted process of precipitation. These colours are 
admixtures of pure normal lead chromate, with either sulphate 
of lead (in which case the resulting colour is still called "pure") 
or with barytes, gypsum, or some other white base. 

Sulphate of lead is produced along with the chromate by 
precipitation, using either sulphate of soda or sulphuric acid 
as the precipitant, while the barytes, &c., is added to the 
colour as it is being made. 

The method of preparing the chromes is this : The lead salt, 
either the acetate or nitrate, is dissolved in a tank, D, Fig. 18, 
with the necessary quantity of water; heat may be used to 
facilitate solution, if thought desirable, but the actual precipita- 
tion is done cold. In another tank. D, Fig. 18, is dissolved either 
bichromate of potash or bichromate of soda and sulphate of soda 
(Glauber's salt). As the commercial products are liable to be 
dirty, the solutions must be either filtered or allowed to settle 
in the tanks, and the clear supernatant liquor only used ; when 
both solutions are ready they are run simultaneously into the 
precipitating tank 0, Fig. 18, where the chrome-yellow is pre- 
cipitated, and is allowed to settle to the bottom of the tank ; 
the clear top liquor is run off by opening one of the plugs, H, 
fresh water run in, the whole stirred up and again allowed to 
settle, and the clear liquor run off. The washing is repeated, if 
thought necessary; for best qualities at least three washings 
should be given. After the washing the pulpy colour is thrown 
on to an ordinary felt filter, or put into the filter-press to extract 
as much water as possible, and then dried in a stove at as low a 
temperature as possible. 

If barytes or gypsum be used to tone down the yellow to the 
required shade it is put into the precipitating tank C with a 
quantity of water, with which it is well mixed, so as to get a 
uniform milky mixture ; then the two precipitating solutions, 
which have been prepared in the tanks D D, are run in, care 
being taken that the contents of tank C are thoroughly well 
agitated all the time ; afterwards the colour may be finished as 
before. If the barytes, &c., is not properly mixed there is great 
liability for it to aggregate in lumps, and the proper shade is 
not obtained; besides the finished colour will be full of white 
masses. 



120 YELLOW AND ORANGE PIGMENTS. 

" Pure " Lemon-yellow. 

Lead acetate or nitrate, .... 100 Ibs. 
Bichromate of potash or soda, . . . 25 ,, 

Glauber's salt (sodium sulphate), . . 35 ,, 

" Pure " Chrome-yellow. 

Lead acetate or nitrate, .... 100 Ibs. 
Bichromate of potash or soda, . , . 30 ,, 

Glauber's salt (sodium sulphate), . . 21 

"Pure" Deep Chrome-yellow. 

Lead acetate or nitrate, . . . . 100 Ibs. 
Bichromate of potash or soda, . . . 35 ,, 

To obtain fine shades it is necessary to keep the lead in excess. 
Now, theory indicates that for 100 Ibs. of lead salt 39 Ibs. of 
potassium bichromate are required; in practice, however, it is 
found that if the full theoretical amount is used then the shade 
of the colour is affected, hence it is not advisable to use more 
than 35 Ibs. ; the above recipes have been based on this amount, 
so that in all, the lead is in excess. The weaker the solutions 
used for precipitating, the finer will the resulting pigment be. 

The shade of the pigment obtained depends upon the propor- 
tion between the two precipitants, bichrome and Glauber's 
salt ; the more sulphate there is present the paler will be the 
shade of yellow obtained, a fact which can be gathered by an 
examination of the above and other recipes (which will be 
given) for preparing the chrome-yellows. In consequence, the 
colour-maker can, by altering the proportions of the two bodies, 
produce almost any shade of yellow he may want. There is a 
direct connection between the quantities of the two bodies and 
that of the lead salt used ; if the latter be kept constant, then, 
if the bichrome is increased, the quantity of Glauber's salt must 
be decreased, and vice versd, so that no loss of material may 
result in making the chromes. 

The following equivalents are given, so that the relative pro- 
portions of the two bodies may be kept correctly when the 
recipes are modified to produce other shades than those given 
above : 

1 Ib. of Glauber's salt is equal to 0'4 Ib. of bichromate of potash. 
1 Ib. of bichromate of potash is equal to 2 '5 Ibs. of Glauber's salt. 
The difference between the values of acetate and nitrate of lead 
is so small that in practical use one may be considered equivalent 
to the other ; and the same remark applies to the bichromates of 



PREPARATION OF CHROME-YELLOWS. 



121 



potash or soda. The above equivalents may be used in this 
way : Suppose it is desired to make a yellow intermediate in 
shade between the lemon- and chrome-yellows given above, then 
2J Ibs. less bichromate must be used than for the chrome-yellow. 
The quantity of sulphate equivalent to 2 J Ibs. bichrome required 
to throw down all the lead is 2J x 2J = 6 ; 6J Ibs. of Glauber's 
salt must, therefore, be added to replace the 2 J Ibs. of bichrome 
taken away. 

Cologne yellow is the name originally given to yellows con- 
taining lead chromate and sulphate, and made by precipitating 
with a mixture of potassium bichromate and sulphuric acid. 
Cologne yellows are generally made in two shades, pale and deep. 



Pale Cologne Yellow. 



Lead acetate, . 
Potassium bichromate, 
Sulphuric acid, 



Deep Cologne Yellow. 



Lead acetate, . 
Potassium bichromate, 
Sulphuric acid, 



100 Ibs. 

17 

18 



100 Ibs. 
25 
13 



They are made in the same way as the chrome-yellows above 
described, and are useful to mix with Prussian blue to make 
greens. 

American Chrome-Yellow. This is made by using alum in the 
place of Glauber's salt ; the product is a fine one, the alumina 
salt seeming to have a beneficial influence on the fineness of the 
precipitate ; still care is required in using alum, as otherwise 
there may be a difficulty in precipitating the pigments. The 
follosving recipes will give some idea of the proportions generally 
used : 





Deep. 


Medium. 


Pale. 


Lead acetate, 


120 


100 


100 


Potassium bichromate, 


20 


20 


20 


Alum, .... 


100 


100 


100 


Barytes, 


40 


60 


80 


Gypsum, . 


... 


... 


80 



Another method of making chromes (which, however, is rarely 
used) is to take white lead, treat with nitric acid so that part 



122 YELLOW AND ORANGE PIGMENTS. 

only of the white lead is dissolved, and then add bichromate of 
potash ; according to the amount of white lead used in relation to 
the other constituents, so will the shade of yellow be ; the fol- 
lowing recipes may be taken as examples: 

For Chrome-Yellow. -Take 200 Ibs. of white lead, mix with 
water to a fine paste, and then add 40 Ibs. of nitric acid of 1 '42 
specific gravity (84 Tw.) ; when all effervescence has ceased 
more water is added, and then a solution of 35 Ibs. of bichromate 
of potash ; the yellow formed is finished in the usual w r ay. 

For a Deep Lemon-Chrome. Take 300 Ibs. of white lead, treat 
with water and 40 Ibs. of nitric acid, as before, and precipitate 
with 35 Ibs. of bichromate. 

A cheaper class of chrome-yellows is made by precipitating the 
chromate of lead on to a white base; barytes, china clay, and 
whiting have been used for this purpose. When well made 
these are quite as good to use as the technically pure chromes 
described above, although the preference is given to the latter by 
most users. 

The following recipes may be taken as standards for the 
preparation of these cheap chromes : 

Lemon Chrome -yellow. 

Lead acetate or nitrate, . . . 100 Ibs. 

Barytes, 400 

Potassium bichromate, . . . 35 ,, 

Chrome-yellow . 

Lead acetate or nitrate, . . . 100 Ibs. 

Barytes, 200 

Potassium bichromate, . . . 35 ,, 

Deep Chrome. 

Lead acetate or nitrate, . . . 100 Ibs. 

Barytes 75 

Potassium bichromate, . . . 35 ,, 

The depth of shade may be altered at will by varying the 
amount of barytes used : an increase in the amount reduces the 
depth of shade, while a reduction in the amount increases the 
depth ; the quantities of the other materials need not be altered. 
The method of working has already been given. 

China clay can be used instead of barytes, and less weight of it 
will be required to produce any given shade than of barytes, 
owing to the china clay being so much lighter in specific gravity, 



PREPARATION OF CHROME-ORANGES AND SCARLETS. 123 

and, therefore, more bulky weight for weight, the relative power 
of the two bodies being as 1 to 2 or 3 ; that is, 1 Ib. of china 
clay will tone down chrome as much as 2J or 3 Ibs. of barytes. 
Gypsum may also be used, and is intermediate between china 
clay and barytes in its toning powers, but it is not so good for 
this purpose as the other two bodies, as there is a reaction 
between the gypsum and lead salts which renders it impossible 
to keep the lead in excess, and so there is a tendency to change 
the shade of yellow which is being made } when nitrate of lead is 
used instead of acetate this action of gypsum is more noticeable. 

Whiting is sometimes used to tone down the yellows ; for pale 
shades its use is not advisable, and, although it may be used for 
deep shades, yet the other whites just named are to be preferred. 
Being somewhat alkaline in its reactions and liable to contain 
traces of caustic lime, from being overheated in the process of 
manufacture, it often has an undesirable tendency to turn the 
shades of the yellows with which it may be mixed to an orange. 

When barytes, china clay, gypsum, or whiting are used as 
toning colours it is advisable to grind them with water to ensure 
perfect admixture with the other ingredients of the yellow. 

2nd. PREPARATION OP CHROME-ORANGES AND 
SCARLETS. The affinity between the chromic acid on the one 
hand and of the lead on the other not being very strong, when 
the chromates of lead are treated with a stronger base, such as 
soda or potash, a part of the chromic acid is eliminated, and a 
basic chromate of lead is formed ; this reaction is expressed by 
the following equation : 

2PbOO 4 + 2NaOH = Na 2 Cr0 4 + PbOPbCr0 4 + H 2 O. 

Normal lead Sodium Sodium Basic lead Water, 

chromate. hydroxide. chromate. chromate. 

The basic lead chromate has a deep red colour ; if the action of 
the alkali only proceeds to a limited extent, then the basic red 
chromate formed combines with the excess of yellow normal 
chromate giving rise to an orange-coloured body, which is a mix- 
ture of the two chromates of lead; the shade of the orange 
pigment thus produced will depend, to a large extent, upon the 
amount of the action which takes place between the normal 
chromate and the alkali. The preparation of the chrome-oranges 
and scarlets depends upon the action of alkaline bodies upon the 
normal lead chromate. As with the yellows no actually chemi- 
cally-pure oranges are made ; the technically-pure colours contain 
more or less sulphate of lead, while the common colours generally 
contain barytes. The orange and scarlet chromes are much 



124 YELLOW AND ORANGE PIGMENTS. 

easier to make than the yellows, as the conditions of making are 
more favourable to the production of orange shades than they are 
of the yellow shades. This is partly owing to the fact that these 
colours are made at a high temperature, while with the yellows 
it is necessary to keep the mass as cold as possible, if good 
yellows are to be obtained. 

1. Pure Chrome- orange. (a) 100 Ibs. of lead acetate, 35 
Ibs. of bichromate of potash or soda, and 9 Ibs. of caustic soda 
(77 per cent.*) are separately dissolved in water; the lead solution 
is run into the precipitating tank, the bichromate run in and 
chrome-yellow precipitated ; this is allowed to settle, the clear 
top liquor run off and then the caustic liquor run on to the 
yellow ; the mixture is heated until the desired shade is obtained; 
the orange is allowed to settle, the top liquor run off, and the 
colour washed with water two or three times, and when dried 
is ready for use. This recipe will give a pure chrome- 
orange. 

(6) A cheaper method of making orange is as follows : 100 
Ibs. of lead acetate, 30 Ibs. of bichromate of potash or soda, and 
21 Ibs. of Glauber's salt are used to make the chrome-yellow in 
the manner described above (p. 119). In another vessel 10 Ibs. 
of quicklime are slaked to a thin cream ; when ready this is 
strained through a sieve, so as to free it from any gritty particles, 
on to the chrome-yellow ; the mass is now boiled (about an hour 
being generally required) until the colour has changed to an 
orange ; the orange is finished as before. 

The first of the above recipes will give a deep orange, the 
second a paler one ; by varying the quantity of lead sulphate in 
the yellow, which is done by using more or less Glauber's salt, 
the shade of orange can be altered at will. The shade is also 
modified by the quantity of alkali used and the length of time 
the colour is boiled ; the more alkali and the longer the boil the 
darker will the orange be. 

2. Common Chrome- orange. (a) Pale Orange. Make a 
yellow as described above from lead acetate 100 Ibs., barytes 
200 Ibs., and bichromate 35 Ibs.; then add 10 Ibs. of quick- 
lime freshly slaked ; boil till the shade has been developed, wash 
and dry the pigment. 

(b) Deep Chrome. Prepare a yellow from 100 Ibs. of lead 

* 77 per cent, caustic soda is nearly chemically pure ; it is made by two 
firms in this country ; if the more common and weaker makes of caustic 
are used, then a proportionately larger quantity of them must be taken ; 
10 Ibs. of 77 per cent, soda are equal to 11| Ibs. of 68 per cent, soda or 
to 13 Ibs. of 60 per cent. 



PREPARATION OF CHROME-RED. 125 

acetate, 75 Ibs. of barytes, and 35 Ibs. of bichromate ; then boil 
with 10 Ibs. of freshly-slaked lime and finish as usual. 

The shade of the orange is modified by the quantity of barytes, 
which may be used in proportion to the other constituents ; the 
amount of alkali used and the length of the boil also have some 
influence. Gypsum or china clay may be used instead of barytes 
if required. 

3. Pure Scarlet-chrome. (a) Dissolve 100 Ibs. of lead salt, 
35 Ibs. of bichrome, and 12 J Ibs. of caustic soda (77 per cent.), 
each separately in water. Add the bichrome solution to the 
lead solution, allow the yellow precipitate to settle, run off 
the clear top liquor, then add the caustic solution and boil up 
the mixture, continuing the boil until the required scarlet shade 
has been fully developed ; then wash, dry, and finish the pigment 
in the usual way. 

(6) 100 Ibs. of white lead are mixed into a paste with water, 
30 Ibs. of bichromate of potash and 12^ Ibs. of caustic soda (77 
per cent.) are dissolved in water, these two bodies being mixed 
together ; when ready the two solutions are mixed and boiled 
until the scarlet colour is developed ; the pigment is now finished 
in the usual way. 

(c) See chrome-red, Runge's process. ^ 

3rd. PREPARATION OF CHROME-RED. This body is 
also known as Persian red, Derby red, American vermilion, 
Chinese red, Victoria red, chrome-red, &c. It is the basic 
chromate of lead, Pb O, Pb Cr O 4 , whose composition and pro- 
perties and principles of manufacture have already (p. 116) been 
dealt with. 

Preparation of Chrome-red. (a) To make this pigment 100 Ibs. 
of white lead are mixed with water into a fine paste ; 50 Ibs. of 
potassium bichromate and 15 Ibs. of caustic soda (77 per cent.) 
are dissolved in water and mixed with the lead, and the whole 
boiled until the colour is properly developed ; the pigment is 
allowed to settle, the top liquor poured off, and the red washed, 
dried, and finished as usual. 

(b) Runges Method. Runge has described the following 
method for preparing chrome-red : 448 Ibs. of litharge are 
intimately mixed with 60 Ibs. of salt (sodium chloride) and 
50 gallons of water ; the mixture is allowed to stand for four 
or five days, stirring at intervals during that time, and adding 
water if the mass exhibits a tendency to set hard ; at the end of 
the time stated a semi-solid greyish mass will be obtained. 
150 Ibs. of bichromate of potash are now added, and the mass 
boiled for about two hours. The pigment formed is finished in 



126 YELLOW AND ORANGE PIGMENTS. 

the usual way. This process has not given good results in the 
hands of the author; a scarlet-red is the deepest colour that 
could be got, which is too pale for chrome-red. 

(c) Prinvalt's Process. The best process which the author 
has tried for preparing chrome-red is that of Prinvalt, which is 
carried out thus : Two equivalents (100 Ibs.) of white lead, one 
equivalent of potassium chromate, K 2 0r0 4 (30 J Ibs. of potassium 
bichromate neutralised with caustic potash), and 50 gallons of 
water are mixed together and allowed to stand for two days ; 
the mixture being stirred up at intervals. The mass is now 
boiled for half an hour or so until the red colour develops ; it is 
allowed to settle, the top liquor run off, and the colour washed 
twice with water and once with weak sulphuric acid (4 Ibs. in 
40 gallons of water), then dried. 

PROPERTIES OP THE LEAD CHROMES. The 
chemical properties of these colours have been already (p. 116) 
described, so that it is only necessary here to deal with them 
so far as their properties as pigments are concerned. 

The lead chromes are pigments of considerable brilliance of 
hue, and their covering power or body is very great, being 
superior to that of all other yellow colours, so that it is very 
Difficult to find substitutes for them. The texture of these 
pigments is usually very fine, but there is considerable difference 
between the products of different makers in this respect. When 
well made they are very durable colours, exposure to all the 
ordinary atmospheric influences heat, light, air, and moisture 
having very little injurious action on them. Sulphuretted hydro- 
gen and sulphur compounds turn them black, owing to the 
formation of the black sulphide of lead ; some samples show a 
tendency to acquire a green hue when mixed with oil, which is 
due to an action between the chromic acid of the colour and the 
oil ; probably in most cases when this occurs the action has 
been set up, in the first instance, by a small quantity of alkaline 
chromate left in the colour owing to defective washing, and this 
having a powerful action upon the oil oxidises it, and, at the 
same time, turns it green ; this action having once been set up 
gradually extends throughout the mass of colour affected. This 
defect is more likely to happen with the oranges, scarlets, and 
reds than with the yellows. 

The chromes are capable of being mixed with a large number 
of other pigments without being themselves affected or affecting 
others. It is not advisable to mix the chromes with pigments 
like ultramarine and cadmium yellow, which contain sulphur, as 
the mixture is apt to take a reddish tone owing to the formation 



ASSAY AND ANALYSIS OP THE LEAD CHROMES. 127 

of a black sulphide of lead. Highly basic colours should not be 
mixed with chromes, as then they turn orange. Chrome-yellows 
do not work well with whiting in distemper work, as they 
sometimes turn orange; but the oranges and reds may be so 
used. 

Chrome-yellow cannot be used along with lime or silicate of 
soda, or other alkaline vehicle, as these will turn them orange. 
The red seems to have a crystalline nature which it will lose by 
friction; in doing so, it turns of an orange colour; care must, 
therefore, be taken not to grind the red too much. 

ASSAY AND ANALYSIS OF THE LEAD CHROMES. 
As there is such a great variation in the chrome colours, and 
one maker's lemon-chrome may differ from another maker's lemon- 
chrome, it is important to assay the chromes for colour, covering 
power, softness, and freedom from grit. The methods of assay 
will be found in Chapter X. Chrome-yellows are rarely adulter- 
ated, in a sense, as there is no cheaper yellow of an equal 
brilliance of colour; the only colour which could be used is 
yellow ochre, but such use is not practicable. An analysis of the 
chrome colours is rarely required ; for a scheme for such an 
analysis reference may be made to the Oil and Colourmaris 
Journal for September, 1888, or to the Chemical News, December 
31, 1886. The following scheme of analysis, based on the one 
there given, gives satisfactory results : 

For Moisture. Weigh out 2 grammes, dry in an air oven at 
110 to 120 C. for a few hours, and, after allowing to cool, again 
weigh; the loss of weight shows the amount of moisture originally 
present. 

For Lead. Weigh out 2 grammes of the chrome, add 10 to 15 
cc. of strong sulphuric acid, heat until the chrome is completely 
decomposed, allow to cool, dilute with water, add a little alcohol, 
filter, wash the precipitate well, mixing the first wash waters 
with the main filtrate, then dry, ignite, and weigh the precipitate 
or residue in a porcelain crucible. This residue, if the chrome 
be pure, consists of lead sulphate, Pb S O 4 ; but if the chrome 
contain barytes or china clay, the residue will also contain those 
bodies. The amount of these, as ascertained by another operation, 
is deducted to find the weight of lead sulphate. This should be 
calculated to lead oxide by multiplying by 0'736. 

For Chromium. Boil the filtrate from the lead until all the 
alcohol has been driven off; then add ammonia in slight excess 
and boil well until the liquor is colourless; then filter, wash, dry, 
ignite, and weigh as usual. The residue on the filter is chromic 
oxide, Cr 2 O 3 , and from its weight the quantity of lead chromate, 



128 YELLOW AND ORANGE PIGMENTS. 

Pb Or O 4 , in the chrome-yellow may be calculated by multiplying 
by 4-23. The amount of lead oxide corresponding to this can be 
calculated by multiplying by 2-9. 

For Lead Sulphate and Barytes. Weigh out 2 grammes, boil 
with strong hydrochloric acid until the chrome is completely de- 
composed ; any insoluble residue is barytes or china clay ; this 
can be filtered off, care being taken to filter while boiling and to 
wash the residue on the filter with boiling water, so as to prevent 
the precipitation of lead chloride from the solution. The residue, 
after drying, is weighed in the usual way. The nitrate from the 
barytes (or the solution in the acid, if there be no barytes) is 
boiled, and then barium chloride is added ; the precipitate of 
barium sulphate is filtered while boiling and well washed with 
boiling water ; it is dried, ignited, and weighed as usual. From 
its weight the amount of lead sulphate in the chrome may be 
calculated by multiplying by 1-3 ; and the amount of lead oxide 
corresponding to this by multiplying the lead sulphate by 0-736. 

If the amount of lead oxide present in the form of chromate 
and sulphate does not equal that of the total lead oxide present, 
the difference may be considered to be present in the form of 
basic carbonate, and the amount of this can be calculated by 
multiplying by 1*16. 

A qualitative analysis should, in all cases, precede the quanti- 
tative analysis, and the method of carrying this out modified 
accordingly. Should whiting be present, its amount may be 
ascertained by dissolving 2 grammes of the chrome in dilute 
nitric acid, filtering off any insoluble residue (which may be 
neglected), then precipitating the lead present as sulphate by 
adding sulphuric acid, and filtering the precipitate off, which may 
also be neglected. To the filtrate is added ammonia in slight 
excess (no precipitate should be obtained ; if there be one, filter 
it off); then add ammonium oxalate, filter, wash, dry. and weigh 
the precipitate, the weight of which at once gives the amount of 
whiting in the chrome. If gypsum (calcium sulphate) be present, 
its amount may be ascertained by dissolving 2 grammes in hydro- 
chloric acid, adding dilute sulphuric acid, allowing to cool, and 
filtering off any lead sulphate, lead chloride, and barytes which 
may be present ; to the filtrate is added ammonia in slight 
excess (any precipitate which may be obtained is filtered off) and 
ammonium oxalate ; the precipitate of calcium oxalate is filtered, 
washed, dried, and, after burning in a crucible, weighed ; from 
the weight of calcium carbonate so obtained that of the calcium 
sulphate present may be calculated by multiplying by 1*36. 

Schemes for the analysis of chromes based on the solution of 



PREPARATION OF ZINC CHROME. 



129 



the lead chroraate present in caustic potash have been devised, 
but the author does not consider that these give as satisfactory 
results as the scheme just described. The section on the 
analysis of Brunswick greens may be consulted. 

ANALYSES OF "PURE" CHROME COLOURS. 





. 












CONSTITUENTS. 


g|a 

&& 


<c 


r a>d 

111 


> 

g ft G 

Hi 


s la 
fill 


111 


S 




" OiO 


O 


i Q 


o 


- OQ 


= ^o 


o 


Water and volatile ) 
matter, . . . ( 


6-900 


2-48 


1-850 


5-640 


3-625 


1-275 


1-69 


Lead chromate, | 
Pb Cr 4) . f 


56-066 


55-30 


87 -834 


73-320 


60-307 


59670 


57-55 


White lead, 2 Pb C 3 1 
+ Pb H 2 2 , . . J 


523 


11-72 


1 -320 


8-372 


35-478 


39-470 


(PI) 
{ 39 99 


Lead sulphate, PbSO 4 , 


36-511 


30-30 


8-455 


11-986 


... 


... 


... 




100-000 


99-80 


99-456 


99-328 


100-410 


100-415 


99-03 



ZINC CHROME. 

This pigment is the chromate of zinc, Zn Cr 4 , which has the 
composition 

Zinc, Zn, . . . . . . 35 "81 per cent. 

Chromium, Cr, 28 '93 ,, 

Oxygen, 0, . . . . . 35 -26 



or 



Zinc Oxide, Zn 0, 
Chromium trioxide, Cr 3 , 



100-00 

44-63 per cent. 
55-37 



100-00 

PREPARATION OP ZINC CHROME. Zinc chrome 
is prepared in two ways 1st, by precipitating a solution of zinc 
sulphate with a solution of chromate of potash ; 2nd, by treating 
zinc oxide with either chromic acid or potassium bichromate. 

1st. By Precipitation. The preparation of zinc chrome by 
precipitating solutions of zinc salts is very difficult, and only by 
the exercise of some care is the operation a success ; this arises 
from the fact that zinc chromate is a substance which, while 

9 



130 YELLOW AND ORANGE PIGMENTS. 

insoluble in water, is readily soluble in acids and alkalies, and 
in various saline solutions. If the solutions used are slightly 
acid the zinc chromes will not form, consequently the acid 
bichromates cannot be used for preparing it. If the solutions 
are alkaline, then there is a tendency for the chrome to be 
decomposed, and for the white hydroxide of zinc to be formed. 

The materials used in the preparation of zinc chrome are the 
sulphate of zinc and the chromate of potash; the chloride of zinc 
may be used or the chromate of soda. The principal points are 
the use of neutral solutions as strong as possible. 

61 J Ibs. of zinc sulphate are dissolved in as small a quantity 
of water as possible, and the solution is boiled ; while boiling, a 
solution of 32J Ibs. of normal sodium chromate in water is 
added, and the mixture boiled for one hour; the zinc chrome is 
precipitated, and can be collected on a filter, washed, and dried 
at a low temperature. The solution of zinc should be neutral ; 
as zinc sulphate is liable to be slightly acid the solution should 
be tested with a piece of paper dipped in a solution of Congo 
red, and if found to be acid, shown by the paper turning blue, then 
the solution should be neutralised by adding sufficient caustic 
soda. The solution of normal sodium chromate can be made by 
dissolving 26^ Ibs. of sodium bichromate in water, heating to 
the boil, and adding sufficient caustic soda to just neutralise the 
bichromate ; this is best ascertained by the use of test paper 
made from a solution of phenolphthalein in methylated spirit, 
the caustic soda being added until a drop of the chromate 
solution taken out just turns the paper a faint pink. The 
solutions are now ready, and can be used as described above. 
The chrome so prepared is of a fine, deep, lemon-yellow colour, 
with a good body. 

The reaction between the zinc and sodium salts is expressed 
in the equation : 

ZnS0 4 + Na 2 Cr0 4 = ZnCrO 4 + Na 2 S 4 . 

Zinc Sodium Zinc Sodium 

sulphate. chromate. chromate. sulphate. 

That with the potassium chromate is similar. 

Another plan of making zinc chrome, and which gives good 
results, is to dissolve 61 i Ibs. of zinc sulphate in water, and to 
add 26 J Ibs. of sodium bichromate dissolved in a small quantity 
of water ; no precipitate will be obtained. The mixture is now 
boiled and, while boiling, a saturated solution of soda crystals is 
added in small quantities at a time, waiting between each addi- 
tion until all effervescence has ceased ; the addition is continued 
until there is little or no effervescence on adding more soda 



PREPARATION OF ZINC CHROME. 131 

crystals ; the whole is now boiled for about one hour, during 
which period the zinc chrome will gradually precipitate, and may 
be filtered and finished in the usual way. Care must be taken 
not to add too much soda crystals or, otherwise, only a precipitate 
of zinc carbonate will be obtained. In both methods potassium 
chromate may be substituted for the sodium salt. 

2nd. From Zinc Oxide. This method was the subject of a 
patent taken out by James Murdoch in 1847, who describes the 
preparation of three shades of zinc chrome. 

1. A Marigold Tint. 125 Ibs. of bichromate of potash are 
dissolved in 70 gallons of water and boiled ; 60 Ibs. of zinc oxide 
are mixed into a thin cream with water and then poured into the 
bichrome solution ; the mixture is allowed to stand for from 
24 to 30 hours ; after which it is boiled up for one hour, allowed 
to settle, and the colour finished as usual. The liquor from the 
colour is kept and used in making the second tint. 

2. A Lemon Tint. The liquor from the last tint, together with 
the first washings, are put into a boiler, and a solution of 75 Ibs. 
of zinc oxide in 44 Ibs. of sulphuric acid of 65 J Tw. is added and 
the mixture is boiled for one hour ; the colour formed is allowed 
to settle, the clear top liquor run off and kept for making the 
third tint, and the colour finished as before. 

3. A Pale Tint. The liquor from the second tint is now mixed 
with a solution of 15 Ibs. of oxide of zinc in 7 Ibs. of sulphuric 
acid, boiled and treated as before. 

Clarke subsequently took out a patent in 1853, in which he 
describes the preparation of two shades of zinc chrome. 

1. A Dark Chrome. 112 Ibs. of bichromate of potash are 
dissolved in boiling water and 70 Ibs. of zinc oxide, previously 
mixed with 35 gallons of water to a thin cream, added, and the 
mixture boiled for one hour ; then the colour is allowed to settle, 
the top liquor poured off and used for making a paler tint (see 
below), and the chrome washed and finished. 

2. Lemon Zinc Chrome. The liquor from the deep chrome is 
boiled down till it has attained a strength of 26 Tw. ; to every 
8 gallons of this liquor 40 Ibs. of zinc oxide, previously dissolved 
in 24 Ibs. of sulphuric acid, are added ; the mass is now boiled 
for one hour, the chrome allowed to settle, and, after decanting 
off the top liquor, washed and finished as usual. 

Instead of potassium bichromate, chromic acid may be used 
with similar results. The following recipes may be used : 

1. A Deep Chrome. Mix 100 Ibs. of zinc oxide into a thin 
paste with 30 gallons of water ; then add, by degrees, boiling the 
whole of the time, 130 Ibs. of chromic acid; after the whole has 

EIJBR^ 

or THE ir 



132 YELLOW AND ORANGE PIGMENTS. 

been added, continue the boiling for one hour longer ; then allow 
the colour which has been made to settle, decant the top liquor 
and wash. 

2. Middle Chrome. Use 200 Ibs. of zinc white and then 
proceed as above. 

3. Pale Chrome. Use 300 Ibs. of zinc oxide, and proceed as 
above. 

With all the processes just described there is great loss of 
chrome, as the liquors which are run off and the washings 
contain chromic acid, both free and in the form of chromates ; 
these liquors are somewhat difficult to utilise ; they might be 
used to make lead chromes, especially the oranges. This waste 
of chromic acid in making zinc chromes by these processes adds 
considerably to the cost. Another defect in the colours thus 
made is that they are very liable to be gritty, especially if the 
excess of chromic acid or alkaline chromate is not thoroughly 
washed out, for, if left in, this tends to crystallise in the colour; 
the presence of this chromate in the colour will lead to the 
colour, when mixed with oil, turning greenish, due to the 
oxidising action of the chromic acid on the oil. 

The lead chrome plant shown in Fig. 18 may be used for making 
the zinc chromes. 

PROPERTIES OP ZINC CHROME. Zinc chrome is a 
yellow pigment of good colour and body ; it is readily soluble in 
acids ; all the common mineral acids dissolve it, even when 
dilute; it is also soluble in many organic acids. Caustic soda or 
potash, when mixed with it in small quantities, decompose it 
with the formation of zinc hydroxide and a chromate of the 
alkali; when in considerable excess they dissolve it. Ammonia 
will dissolve zinc chrome. Heat turns it a grey-violet tint, due 
to its decomposition into the oxides of zinc and chrome. 

As a pigment it is quite permanent, resisting exposure to light 
and air when well made ; while sulphureous gases do not affect 
the colour. It can be mixed with all other colours without 
being affected by them in any way. 

Zinc chrome has usually a good colour and body, although in 
this respect it is scarcely equal to the lead chromes. 

ASSAY AND ANALYSIS OP ZINC CHROMES. In 
assaying zinc chromes the points to be noted are: 1st, colour; 
2nd, covering power; 3rd, texture or freedom from grit, so as to 
have a soft feel; these points can be tested for in the usual way. 

Zinc chromes are liable to adulteration with the lead chromes, 
as these are rather cheaper ; or to form the pale shades the deep 
shades may be let down with a cheaper white base than zinc 



OCHRES AND SIENNAS. 133 

oxide. Pure zinc chrome is soluble in acetic acid without 
effervescence ; any yellow residue indicates adulteration with 
lead chrome or yellow ochre, which can be distinguished by the 
application of special tests. A white residue indicates adulter- 
ation with barytes, china clay, &c. ; while admixture with either 
whiting or white lead will be indicated by effervescence with 
the acid. Zinc chrome should not be discoloured on adding 
to it a little ammonium sulphide. 

A sample of zinc chrome analysed by the author had the 
composition : 

Water, 16 '08 per cent. 

Zinc chromate, 3S*99 

Zinc oxide, 4'93 ,, 



100-00 

LEMON CHROME. BARIUM CHROME. 

This pigment is the chromate of barium, Ba Cr O 4 , prepared 
by precipitating a solution of barium chloride with potassium 
bichromate. Its use as a pigment has become obsolete owing to 
its having only a very pale yellow colour and to its want of 
body. The lemon chromes made from lead are much brighter 
and have more body. 

OCHRES AND SIENNAS. 

Next to the chromes the ochres and siennas are by far the 
most important of the yellow pigments. They form a group of 
natural pigments of inorganic origin, found in comparatively 
large quantities in many parts of the world. The ochres are 
generally of a good yellow colour, varying from a faint brownish 
to a reddish tint of yellow ; the siennas (which are so called 
because they were first found near the town of Sienna, in Italy) 
are of a brownish-yellow tint, varying somewhat in depth of 
shade. 

Ochres occur in rocks of all geological ages ; in the Oolitic 
rocks of Oxfordshire, the Mountain limestone of Derbyshire, the 
Silurian slates of Wales, the Granites of Cornwall, the Liassic 
and Cretaceous rocks of France, and in the volcanic rocks of 
Italy ; so that geological age has no connection with their for 
iiiation. 

Ochres and siennas consist essentially of an earthy base, 
coloured, in the case of the ochres, by the hydrated ferric oxide ; 
in the case of the siennas, by the hydrated ferric oxide and 



134 YELLOW AND ORANGE PIGMENTS. 

manganese oxide. The character of the base varies according to 
the locality where the pigment is found. The analyses given 
below illustrate this point very well. 

As stated above, these pigments owe their colour to hydrated 
peroxide of iron ; so far as can be judged from the composition 
and manner in which they are found, this oxide of iron is, in all 
cases, an alteration-product, formed by the oxidation of ferru- 
ginous minerals, especially pyrites, and, in the case of siennas, 
of manganiferous ores. The manner of occurrence of ochres and 
siennas, so far as information at hand is available, appears 
to be different; in the case of ochres, these seem to have been 
formed by the decomposition and oxidation of iron minerals at 
the spot where the pigment is found, for, in the majority of 
cases, the ochre occurs more or less mixed with the gangue or 
matrix, from which mineral it originated. The analyses of the 
Welsh and Derbyshire crude ochres given below are examples 
of ochres formed in this manner. This occurrence of much 
matrix or gangue in crude ochres necessitates the adoption of a 
method of levigation to prepare them for use as pigments. 

Siennas occur under various conditions, generally as aqueous 
deposits, in hollows and basins ; some of these are of recent 
formation, since bronze idols and other articles of human 
manufacture have been found in them. Siennas seem to 
have been formed by water draining over beds of iron and 
manganese ores, so as to become charged with oxide of iron and 
organic matter, and then flowing into a hollow in which the 
iron, &c., it contained was deposited ; in process of time the 
hollow was tilled up and the bed of sienna formed. Siennas 
differ from ochres in containing very little extraneous matter, 
and, therefore, they require little beyond grinding to fit them 
for painting, when they are known as "raw sienna" (see p. 141). 

Ochres are prepared for use as pigments by grinding and 
levigating. The plant used for this purpose varies at different 
works, its construction being largely dependent upon the nature 
of the ochre which is being treated. Some ochres are soft and 
powdery, these only require levigation; while others are harder, 
and need to be ground before they can be levigated. Fig. 28 
shows one form of this plant, and in Fig. 12 is shown another 
form used in the china-clay industry, which may also be used 
for ochres. A is a large tank which, in Derbyshire and Corn- 
wall, is known as the "buddle"; in this is placed the rough 
ochre, just as it comes from the ground, if the nature of the 
material permits; or it may be subjected to a preliminary 
grinding in an edge runner mill. In the buddle it is 



OCHRES AND SIENNAS. 135 

thoroughly mixed with a constantly flowing gentle current 
of water, which carries off the finer particles of the ochre, 
and leaves the very coarse material behind ; as the latter 
accumulates in the huddle it is removed from time to time 
and thrown away. From the buddle the water carries the 
ochre into a settling-tank, placed at a rather lower level, where 
the coarser particles of ochre settle out; when they have 
accumulated sufficiently the water is run out of the tank, and 
the ochre removed. From the first settling-tank the water, 
which still contains some ochre, is run into a second tank 
rather larger than the first, so that the current of water becomes 
more sluggish, and the finer particles of ochre subside more 
readily ; in this second tank the ochre is allowed to settle until 
enough has accumulated, when it is removed as before. From 
the second tank the water runs off into a third tank, in which a 
still finer quality of ochre settles out. All these tanks are 
shown in Fig. 28 ; sometimes a fourth tank is arranged. The 
number of settling-tanks much depends upon the quality of 
crude material and the variety of grades of ochre it is desired 
to make ; the grade of ochre which settles out in each tank is 
different from the others ; that in the first is darker in tint and 
coarser in quality than that which settles out in the third ; 
while, if a fourth tank is used, this will give a pale and fine 
quality of ochre. It will be found best in large works to 
arrange for three sets of settling-tanks, arranged side by side ; 
the material from the buddle is run into the first of these 
sets until the tanks are full ; then the stream is turned into the 
second set, which are also filled; then the stream is turned into 
the third set ; by this time the ochre in the first set will have 
settled out, and the tanks can be emptied and got ready for the 
water to be turned into them. When the third set has become full 
the second set will be ready for emptying, and this is done ; 
then the third set is emptied ; thus there is always one set being 
filled, another settling out, and the third being emptied. There 
is scarcely two ochre works where the arrangement of the 
levigating plant is alike, as much depends upon the locality, the 
amount of water at disposal, and the quantity of material being 
treated. 

After being taken out of the settling tanks the ochre is in the 
form of a paste which, as it contains probably 50 per cent, of 
water, must be dried. The arrangements for this vary very 
much at different works ; at some, long horizontal flues are built 
with a fireplace at one end, and the flues covered with flagstones 
on which the wet ochre is placed ; at others, the ochre is placed 



136 YELLOW AND ORANGE PIGMENTS. 

in earthenware pans in an ordinary drying stove ; but in every 
case, the drying of the ochre at as low a temperature as possible 
is important, as too high a temperature turns the colour of the 
ochre rather reddish. 

PROPERTIES OF OCHRES AND SIENNAS. So far 
as their properties as pigments are concerned, the ochres and 
siennas rank among the most permanent pigments at the disposal 
of the painter. They are unaffected by admixture with any 
other pigments, do not act injuriously upon other pigments, and 
are scarcely affected by exposure to the atmosphere and its de- 
structive influences. They work well with all kinds of vehicles, 
and can, therefore, be used in any kind of painting oil, water, 
distemper, fresco, &c. 

Ochres and siennas vary very much in tint, brightness of 
colour, and strength. Oxford ochre is the brightest of the ochres 
and is of a fairly bright brownish-yellow colour. Siennas are of 
a brownish-yellow colour varying much in depth of tint or shade. 
Welsh ochres are rather duller than Oxford ochres ; French 
ochres are moderately bright ; Derbyshire ochres are reddish in 
tone and are darker than other varieties of ochre. They vary 
very much in texture ; Oxford ochre and the siennas are of a 
soft texture ; some are gritty in feel, while others have a clayey 
feel. In body or opacity these pigments vary much. The Oxford 
ochre and the siennas are rather transparent, and are commonly 
used as glazing colours ; the other ochres are more opaque and 
have good body ; hence, they are largely used as body colours, 
especially in house painting. 

The colour of ochres is due to the presence of hydrated per- 
oxide of iron, while siennas also contain small quantities of 
manganese ; the shade or tint depends mainly upon the propor- 
tion of iron and manganese present, and also, but to a less extent, 
upon the degree of hydratiou of the oxide of iron ; in proportion 
as the iron oxide is less and the hydration greater, the yellower 
and brighter the shade of colour ; when the proportion of non- 
hydrated oxide of iron is large the shade becomes redder. When 
ochres are treated with hydrochloric acid, the iron they contain 
is nearly all dissolved out, and yields a yellow solution which 
will give the characteristic tests for iron, while a more or less 
insoluble residue is left behind. 

Heat turns ochres a red colour, the shade of which depends 
upon the temperature and length of time the heating is carried 
on ; these red colours are sold as Venetian red, light red, Indian 
red, <tc. ; their preparation and properties have already been 
described (see p. 105). Siennas are converted by heat into a 



COMPOSITION OF OCHRES AND SIENNAS. 137 

reddish-orange pigment, known as burnt sienna (see p. 141 ). This 
change of colour is due to the passage of the iron oxide from the 
hydrated to the anhydrous condition, but the reason why ochres 
should give reds and the siennas orange is not known. 

ASSAY AND ANALYSIS OF OCHRES AND SIEN- 
NAS. (a) Crude Ochres and Siennas. These should be 
assayed for, first, the actual quantity of colour present and, 
second, for the tint or shade of the colour it gives. This last can 
be done in the usual way; the first can be ascertained as follows: 
A tall glass of a conical shape is provided ; a glass funnel with a 
long stem passes down to the bottom of the glass into which is 
put about 25 to 30 grammes of the crude ochre ; into the glass is 
now passed a gentle current of water sufficiently strong to carry 
out of the glass all the finer particles of colour while leaving the 
heavier and more gritty particles behind, which are collected by 
filtering and, after drying, are weighed in the usual way ; from 
the weight is calculated the proportion of colour and grit. Thus, 
the sample of crude Irish ochre (an analysis of which is given 
below) assayed in this way, was found to contain 

Grit, 30-24 per cent. 

Colour, . . . . 69-76 

(b) Prepared Ochres. These only need assaying for colour 
and covering power by the usual methods. 

It is rarely that an analysis of ochres and siennas is required ; 
but analyses of several varieties are given below, which show 
their constituents and what to look for in analysing them. 
Ochres are rarely, if ever, adulterated. Ochres which are 
naturally poor in colour sometimes have a little chrome-yellow 
added to them to bring up the tint ; such an addition may be 
recognised by treating the ochre with hydrochloric acid and 
alcohol, when a green-coloured solution containing chromium 
will be obtained, and the chromium in which may be detected 
by the usual tests. 

COMPOSITION OF OCHRES AND SIENNAS. Most 
of the following analyses have been made by the author; others 
are quoted from various published analyses. The notes appended 
to some of them will be found useful and of interest as showing 
some indications of the origin of these pigments. 

1. Oxford Ochre. The ochres from Oxfordshire have long 
had a reputation for their quality, exceeding, as they do, all other 
ochres in the brightness of their colour and depth of covering 
power. Most of the ochre is found in pits at Shotover, near 



138 



YELLOW AND ORANGE PIGMENTS. 



Oxford, of which the following section is given in 
Dictionary " : 

1. Summit of hill, highly ferruginous grit, . 6 feet. 

2. Grey sand, 3 ,, 

3. Ferruginous concretions, . . . 1 ,, 

4. Yellow sand, 6 ,, 

5. Cream-coloured loam, . . . . 4 ,, 

6. Ochre, 6 inches. 



lire's 



Oxford ochre contains 

Water, hygroscopic, 
Water, combined, 
Calcium oxide, Ca O, 
Sulphur trioxide, S 3 , 
Alumina, A1 2 O 3 , . 
Ferric oxide, Fe 2 O 3 , 
Silica, Si 2 , . 



6-887 percent. 

8-150 

0-998 

1-321 

6-475 
12-812 
63-478 

100-121 



The layer of ferruginous concretions is, probably, the source of 
the colouring matter of this ochre, while the clay which under- 
lies the layer of ochre is the source of the base of the pig- 
ment. 

2. Welsh Crude Ochre. The exact locality from whence 
this sample was derived is not known ; it has a fairly good colour 
and covering power. It contains 



Water, hygroscopic, 
Water, combined, 
Sulphur trioxide, S O 3 
Silica, Si 2 , 
Alumina, A1 2 3 , 
Ferric oxide, Fe 2 3 , 
Copper sulphide, Cu S, 



2-000 per cent. 

12-500 ,, 

1-315 

29-725 

33-315 ,, 

20-705 

515 



100-075 



Some of the iron oxide, 0-765 per cent., exists in a soluble form, 
probably as sulphate, for there is 0-555 per cent, of soluble 
sulphur trioxide. This, and the fact that there is copper sulphide 
present, indicate that this ochre has been formed by the de- 
composition of a cupreous pyrites, which supposition is further 
strengthened by the fact that small pieces of pyrites may be 
picked out of the crude ochre. This ochre requires well 
levigating to get rid of the pyrites as this body would introduce 



COMPOSITION OF OCHRES AND SIENNAS. 139 

an element of change which may exercise an injurious influence 
upon the permanent properties of the ochre. 

3. Irish Crude Ochre. This sample contains 

Water, hygroscopic, .... 9'050 per cent. 

Water, combined, .... 12 '000 ,, 

Insoluble matter, .... 32 '502 
Sulphur trioxide, S 3 , . . 2 '685 



Alumina, A1 2 3 , 
Ferric oxide, Fe 2 3 , 
Calcium oxide, Ca 0, 
Copper oxide, Cu 0, 



16-770 
26-381 

258 
630 

100-260 



The insoluble matter consists of silica and some gritty matter. 

Of these constituents a portion (U'33 per cent, of sulphur 
trioxide, 0-191 of ferric oxide, 0-118 of calcium oxide, and 0*228 
of copper oxide) was soluble in water in the form of sulphates, 
the iron being in the ferrous form ; this points to the fact that 
this ochre was formed by the oxidation of the cupreous pyrites 
existing, disseminated through a siliceous mineral matrix which 
was broken up by the oxidation, and part of which went to form 
the base of the ochre, while much of it exists in the form of 
gritty angular pieces which must be separated before the ochre 
is fit to use as a pigment. 

4. Derbyshire Crude Ochre. This is of a reddish colour 
and is found incrusting masses of pyritous minerals and barytes, 
from which the ochre is separated by levigation. This ochre, like 
the others just noticed, has been formed by the oxidation of 
pyrites, small fragments of which, in an unchanged condition, 
are disseminated through the crude ochre. This sample contained 
more iron than any other examined by the author, as the 
following analysis shows : 

Water, hygroscopic, .... 5*400 per cent. 

Water, combined, .... 6 '000 

Alumina, A1 2 3 , . . . . 1 -040 

Ferric oxide, Fe 2 3 76 '081 

Calcium oxide, Ca O, . . . . 0'561 

Sulphur trioxide, S 3 , . . . 1*744 

Pyrites, FeS, 4*783 

Silica, Si 2 , 1 d ., Q4 
Barytes, BaS0 4 ,j ' 

100-000 

5. Derbyshire Prepared Ochre. This sample is of a reddish 



140 



YELLOW AND ORANGE PIGMENTS. 



shade, not so dark as the last. 
is good. 

Water, combined, 
Barium sulphate, Ba S 64, . 
Silica, Si 2 , 

Calcium sulphate, Ca S 0-4, . 
Calcium carbonate, Ca C O 3 , 
Alumina, Al 2 O 3 , 
Ferric oxide, Fe 2 3 , . 
Magnesia, MgO, 



The covering power of this ochre 



6-100 per cent. 
20946 
4-530 



21 -755 
10-655 
33-498 
trace 

100-000 



6. Cornwall Prepared Ochre. This ochre is 
brownish-yellow shade and has not much covering 
contains 



of a pale 
power. It 



Water, hygroscopic, 
Water, combined, 
Silica, Si 2 , 
Alumina, A1 2 3 , 
Ferric oxide, Fe 2 3 , 
Calcium oxide, Ca O, 



1 '40 per cent. 
10-00 
59-67 

9-72 
18-54 

0-23 

99-56 



7. French Prepared Ochre. The exact locality from which 
this was obtained is not known to the author, but it is usually 
of a bright brownish-yellow colour with a good covering power. 

Water, hygroscopic, . . . . 1 '80 per cent. 
Water, combined, . . ^ 9 '20 

Silica, Si 2 , 54 '00 

Alumina, A1 2 O 3 , .... 13'75 

Ferric oxide, Fe 2 3 , . . . .20*73 
Calcium oxide, Ca O, .... 0'19 



99-67 

8. South Australian Ochres. The following analyses of 
South Australian ochres are quoted in the Journ. Socy. Chem. 
Ind., 1889, p. 313: 

Per cent. Per cent. Per cent. 
1-82 1-92 0-21 



Water, hygroscopic, 
Water, combined, 
Silica, SiO 2 , 
Ferric oxide, Fe 2 3 , 
Alumina, A1 2 3 , . 



6-48 
41-20 
38-40 
12-56 



Per cent. 
1-92 
7-60 
56-60 
11-68 
19-12 



4-00 
6520 

5-76 
21-74 



100-46 96-92 96-91 



BURNT SIENNA. 



141 



9. Siennas. The following are some analyses of raw siennas, 
which are of Italian origin the first and second from the 
neighbourhood of Rome ; the locality from whence the third 
came is unknown. The Roman siennas are found in hollows 
on hill sides, which hollows are now filled up with deposits of 
but, at one time, were the site of small ponds into 



sienna 



which flowed streams highly charged with iron and manganese, 
from deposits of those materials situated above the ponds. The 
rocks of the district consist chiefly of trachyte and granite, 
charged with ferriferous and manganiferous minerals, which 
are the source of the colouring matter of the sienna. 



Water, hygroscopic, 

Water, combined and organic 

matter, 

Silica, Si 2 , . 
Calcium, carbonate, Ca C 63, 
Alumina, A1 2 3 , 
Manganese, Mn 2 , 
Ferric oxide, Fe 2 2 , 
Magnesia, Mg 0, 



Dark. 
Per cent. 

17-550 



Pale. 
Per cent. 

8-250 



Per cent. 
12-400 



9-000 
22-656 
0-960 
2-840 
1-190 
45-820 


11-000 
17-406 
1-075 
5-177 
0-627 
57-032 


9-400 
5-016 
4-460 
7-265 
1-465 
59-965 
trace. 



100-016 100-567 99-701 



It may be assumed that the shade of the siennas varies with the 
amount of manganese it contains, as is shown by the analyses 
given above. 

The commercial value of an ochre depends upon its colour and 
body ; those which excel in these points naturally commanding 
the best prices. The following is approximately the order of 
the various ochres as regards price : Oxford, 100; French, 33 ; 
Derbyshire and Welsh, 25 ; Irish and Devon, 20. 



BURNT SIENNA. 

The siennas are sold in two forms, raw and burnt ; the first 
has already been dealt with and the latter will now be described. 
Burnt sienna is prepared by calcining the raw sienna at a 
moderate red heat until it has acquired the desired shade. The 
tint of the burnt sienna depends not only upon the temperature 
used and the length of time it is exposed to heat, but, also, upon 
the shade of the raw sienna used. Burnt sienna is a pigment of 
a reddish-orange shade, very similar to that of the coal-tar colour 
known as Bismarck-brown. It is very transparent, and is, there- 
fore, mostly used as a glazing or tinting colour by painters and 



142 YELLOW AND ORANGE PIGMENTS. 

artists. It is sold in the form of small pieces, and of a paste 
ground up with water or oil. The former variety is very 
difficult to grind. 

The composition of burnt sienna naturally resembles that of 
raw siennas, only that the heat has driven off most, if not all, the 
water the latter contains. The following analysis will serve to 
show the composition of burnt sienna : 

Water, hygroscopic, . . 9 '450 per cent. 

?d, . 



Water, combinec 

Silica, Si 2 , 

Calcium carbonate, Ca C O 3 , 

Manganese, Mn 2 , 



3-275 
36-912 
1-233 

traces. 



Alumina, A1 2 3 , .... 3 '480 
Ferric oxide, Fe 2 3 , . . . . 45-650 



lOO'OOO 

The whole of the water in this sample had not been driven off 
by the burning. Why raw siennas should give an orange-red 
pigment on calcining and ochres a red is somewhat uncertain ; 
probably the fact that siennas contain organic matter and that 
the iron is in both the ferrous and ferric conditions may have 
some influence. 

MARS COLOURS. 

Under the generic name of Mars colours the late George Field, 
a noted colour manufacturer, introduced a series of yellows, 
oranges, reds, and violets, owing their colour to ferric oxide. 
Field did not publish any account of the method by which he 
produced these colours ; but descriptions of similar products have 
been given by various French and German writers on pigments. 
These colours present no advantage over ochres and iron-reds as 
regards permanency or brightness of tone, but have disadvantages 
as regards cost. 

Mars yellow is made by taking equal weights of ferrous 
sulphate and alum, and adding a solution of carbonate of soda, 
thereby precipitating the iron and alumina ; the precipitate is 
collected, washed well with water, and dried slowly. 

Mars orange is made by slightly calcining the yellow. 

Mars red is made by calcining the yellow at a red heat. 

Mars violet is made by calcining the yellow at a white heat. 

By using milk of lime instead of the soda salt the colours 
could be made cheaper, a plan which is in use for making 
some forms of iron-reds (see p. 106). 

Mars brown was made in a similar manner from a mixture 
of ferrous sulphate, alum, and manganese chloride. 



NAPLES YELLOW. 143 

Mars colours can be distinguished from the ochres and ochre- 
reds by being soluble in strong hydrochloric acid, and by 
containing a large proportion of alumina, but no silica. 

TURNER'S YELLOW. 

Turner's yellow (so named after the inventor, James Turner), 
or patent yellow (from its having been patented in 1781) was at 
one time largely used ; but since the introduction of the chrome- 
yellows it has been gradually, and, perhaps, entirely abandoned. 
It has been known as Montpelier yellow, Cassel yellow, Kassler 
yellow, Verona yellow, mineral yellow, and, probably, by other 
names. 

It is essentially an oxychloride or basic chloride of lead. It 
is made by mixing two parts of litharge and one part of salt with 
water to a thin paste and allowing the mixture to stand for 24 
hours, stirring at intervals ; at the end of this time it will, as a 
rule, have a white colour ; if it has not, more water must be 
added and the mixture again allowed to stand for another 24 
hours or until it becomes white ; it is now washed (to free it 
from alkaline salts), dried, then put into a crucible, and calcined 
at a gentle heat sufficient to melt the mass. The shade of colour 
depends upon the temperature and duration of the heating ; 
usually small samples are taken out of the crucible from time to 
time, and when the right shade has been obtained the contents 
of the crucible are allowed to cool, after which they are ready 
for use. Sal ammoniac may be used in the place of salt. 

Another method of preparation consists in precipitating a 
solution of lead with hydrochloric acid, collecting the precipitate, 
and washing and calcining the lead chloride so obtained; but the 
result is not so good as that obtained by the process above 
described. 

Turner's yellow is met with in many shades of yellow, from a 
fairly bright yellow to a dark orange-yellow ; usually it is in the 
form of heavy, glassy-looking masses, which are rather difficult to 
iirind. It has a good body or covering power, and can be used 
either as an oil- or water-colour. It is not a permanent colour, 
being affected by exposure to light and air and to sulphureous 
gases, which turn it brownish-black. 

NAPLES YELLOW. 

Like the last this is a lead colour and has been superseded by 
the chromes. Naples yellow is a compound of the oxides of 



144 YELLOW AND ORANGE PIGMENTS. 

antimony and lead, and can be prepared of various shades and 
from different materials. 

(a) 1 part of tartar emetic, 2 parts of lead nitrate, and 4 parts 
of salt are intimately mixed together, and the mixture placed in 
a crucible and heated to fusion, at which point it is kept for 
2 hours ; after which the fused mass is treated with water to wash 
out the soluble alkaline salts present in it, and the pigment is 
dried at a gentle heat. 

(6) 1 part of tartar emetic, 2 parts of red lead, and 4 parts of 
salt are treated as above. 

(c) 3 parts of antimony, 1 part of zinc oxide, and 2 parts of red 
lead are heated to fusion in a covered crucible for 4 hours; after 
which they are ground under water and the pigment dried at a 
gentle heat. 

(d) A process of preparing an antimony-lead yellow from the 
dross of lead refining was patented in 1858, by Dick, which con- 
sisted in mixing this dross (which is a mixture of the oxides of 
lead and antimony with some small quantities of other impurities) 
with salt, fusing the mass for 2 to 3 hours, then washing it well 
with water and drying the pigment. 

(e) A yellow not unlike Naples yellow has been made from the 
three oxides of tin, lead, and antimony, by calcining for 3 to 4 
hours in a crucible a mixture of 2 parts of levigated crude anti- 
mony, 2 parts of tin ashes, and 5 parts of white lead ; or 1 part 
each of tin ashes, litharge, and antimony may be used. 

(f) 1 part of type-metal, '2 parts of potassium nitrate, and 
4 parts of salt are fused together, and the fused mass treated as in 
process a. 

(g) Processes for preparing antimony yellows were patented 
by Hallet and Stenhouse, in 1861, as follows: 1. Antimony ore 
was calcined and then mixed with oxide of zinc and litharge, the 
mixture being fused. 2. A mixture of type-metal and zinc oxide 
are fused together. 

The yellows made by the above methods have been sold under 
various names Naples yellow, Jaime, solid yellow, antimony 
yellow, &c. They were rather favourite colours at one time with 
artists, but their use has become nearly obsolete. They are 
bright colours, although not equal to the chromes in this respect; 
are fairly fast to light, but, like all lead colours, are affected by 
sulphureous gases ; iron has an injurious effect upon these 
colours so that they cannot be ground in iron mills with safety. 
They are equally useful as oil- or water-colours, and are of good 
body or covering power. 



KING'S YELLOW. 145 



KING'S YELLOW. 

This yellow, which at one time was in extensive use, is the 
trisulphide of arsenic, As. 2 S 3 . It is found native as the mineral 
orpiment, which is sometimes ground up and used us a pigment. 
The artificial colour is usually made by precipitation, but it can 
also be made by sublimation. 

1. By Precipitation. (a) Arsenic is dissolved in hydrochloric 
acid and a current of sulphuretted hydrogen gas passed through 
the solution ; a fine yellow precipitate of the colour is obtained, 
which is collected and dried at a gentle heat. (6) A fine yellow 
pigment, formerly sold under the name of Royal yellow, is made 
by mixing 2 parts of barium sulphate with 1 part of charcoal and 
calcining the mixture, when barium sulphide is formed ; this is 
ground with orpiment and water into a fine paste, and by boiling 
with water a sulpho-arsenite of barium is obtained ; to this is 
added dilute sulphuric acid which precipitates a mixture of 
barium sulphate and sulphide of arsenic as a fine yellow colour, 
which is collected, washed, and dried. 

2. By Sublimation. 1 Ib. of sublimed sulphur and 2 Ibs. of 
white arsenic are thoroughly mixed together and placed in a 
crucible ; this is covered with another crucible or with a special 
condenser, and heated in a furnace. The arsenic and sulphur 
react and form the sulphide, which, being volatile, sublimes into 
the cover, and is collected, washed, and dried ; it varies a little 
from time to time in shade. 

King's or arsenic yellow is a very bright pigment, almost 
rivalling the chromes in beauty. It has good body and works 
well either in oil or water, but is not a durable colour, as exposure 
to light causes it to fade, while air and moisture have no action 
on it. It does not mix well with other pigments, since, when 
associated with lead pigments, or with verdigris, emerald green, 
or other copper pigments, it gradually acquires a dark brownish 
tint owing to the formation of the black sulphides of lead and 
copper. It can be mixed with ultramarine, cadmium yellow, or 
oxide of iron without change. Being an arsenic colour it is very 
poisonous, and, therefore, its use is not to be recommended. 
Partly in consequence of this objection it has become nearly, if 
not entirely, obsolete. King's yellow forms a colourless solution 
with strong hydrochloric acid ; as also with caustic soda, from 
which, on adding acid, the pigment is reprecipitated. The 
presence of arsenic may be tested for by means of Marsh's test, 
described in the section on emerald green (p. 171). 

10 



146 YELLOW AND ORANGE PIGMENTS. 



REALGAR, ARSENIC ORANGE. 

Realgar is a native arsenic disulphide found in small quantities 
in various localities. It is prepared artificially by a process of 
sublimation as follows : (a) A mixture of 8 Ibs. of white arsenic 
(arsenious oxide) and 4 Ibs. of flowers of sulphur are heated in a 
crucible as in making orpiment. (6) A mixture of 30 Ibs. of 
arsenic, 20 Ibs. of flowers of sulphur, and 40 Ibs. of charcoal is 
made ; a charge of 60 Ibs. of this mixture is heated at a time in 
earthenware crucibles so arranged that the product which sub- 
limes can be collected. This sublimate is then remelted to form 
the colour. Realgar has the same properties as orpiment. 

INDIAN YELLOW. 

Indian yellow or Purree is a most curious product. It has long 
been used in India, but is of comparatively recent introduction 
in this country, where its use is limited. It is made exclusively 
at Monghyr in Bengal by a caste of people known as the Gwalas. 
It is made from the urine of cows fed upon the leaves of the 
mango tree, which food increases the secretion of bile and the 
excess passes into the urine to which it imparts a strong yellow 
colour. The flow of the urine is stimulated by the Gwalas gently 
rubbing the urinary organs two or three times a day ; indeed, 
the cows are so habituated to this that they are unable to pass 
the urine themselves ; the feeding with mango leaves is so 
injurious that its long continuance causes the death of the cows, 
and grass, &c., is occasionally substituted for them ; the average 
life of these cows is from six to seven years. The urine as it 
comes from the cows is collected, and each evening it is boiled 
down in earthen vessels when the yellow is deposited; it is 
gathered on calico, made into balls and sent into the market for 
sale. The annual production is said to be about 100 to 120 cwts. 

Indian yellow is a fairly bright yellow pigment, and is sold in 
the form of small round balls ; it is non-poisonous and has a good 
colouring power ; unfortunately it is not durable, as exposure to 
light soon causes it to fade. Authorities differ somewhat upon 
the composition of Indian yellow, but most agree that it is a 
compound of a peculiar acid known as euxanthic acid (which 
exists in the purree) combined with magnesia ; there is also 
present potassium benzoate and other bodies. The acid itself 
generally crystallises in small needles of a pale yellow colour ; 
it is slightly soluble in cold water, more readily in boiling 



CADMIUM YELLOW. 147 

water, and freely in ether and alcohol. On being sub- 
jected to dry distillation a yellow body, called euxanthone, 
sublimes. The salts of euxanthic acid are all yellow-coloured 
bodies ; those of the alkalies are soluble in water, while those of 
most of the metals are insoluble and may, therefore, be used as 
pigments. 

CADMIUM YELLOW. 

This important yellow pigment, so much used by artists on 
account of the brilliance of its colour and its permanence, is the 
sulphide of the metal cadmium and is composed of 

Cadmium, 77 '78 percent. 

Sulphur, 22-22 

and has the formula Cd S. It is made by passing a current of 
sulphuretted hydrogen gas through solutions of cadmium salts, 
as shown in the equation : 

CdCl 2 + H 2 S = CdS + 2HC1 

Cadmium Sulphuretted Cadmium 
chloride. hydrogen. sulphide. 

G. Buchner has investigated the properties of cadmium yellow 
more thoroughly than any other chemist. He describes * four 
modifications of cadmium sulphide which he distinguishes as 

1. A-modification, obtained by passing a current of sulphur- 
etted hydrogen gas through a slightly acid solution of a cadmium 
salt. The colour is a very bright and pure citron-yellow, has a 
good body, and works well as an oil-colour. By various means 
it can be converted into the B-modification. When used as an 
oil-colour it is quite permanent, but when used in water, or kept 
in a moist atmosphere, it gradually undergoes oxidation, passing 
into sulphate, this change being accompanied by a loss of colour. 

2. B-modification. This has a bright vermilion-red colour, 
and is obtained by passing sulphuretted hydrogen gas through a 
strongly acid solution of a cadmium salt. It is the most 
permanent form of cadmium sulphide and is unaffected by 
exposure to light and air. 

The author has been unable to prepare this red variety of 
cadmium yellow. Although Buchner does not give any clear 
description of the method of obtaining it, yet from the remarks 
as to the conditions under which this red variety is formed, it is 
evident that it cannot be obtained by precipitation free from the 
yellow variety, and that the process of separation consists in 

* Chemiker Zeitung quoted in Journal Socy. Chem. Ind., VI. 665. 



148 YELLOW AND ORANGE PIGMENTS. 

exposing the mixture of yellow and red sulphides to light and 
air for some considerable period. It is not practicable to prepare 
this variety as a commercial article. 

3. C-modification. A variety soluble in water, of no practical 
interest, and prepared by a process of dialysis. 

4. D-modification. This variety has a pale yellowish colour 
and little or no body. It is prepared by passing sulphuretted 
hydrogen gas through an ammoniacal solution of a cadmium 
salt. It is of no practical use. 

Cadmium yellow is made commercially in various shades of 
yellow and orange, the processes for the production of which are 
described below. 

Preparation of Yellow Cadmium. This is prepared in several 
ways. 1. A slightly acid solution of any cadmium salt is pre- 
pared and through it is passed a current of sulphuretted hydro- 
gen gas; the apparatus shown in Fig. 17, p. Ill may be used. 
This has a pure chrome-yellow shade. 2. A lemon-yellow shade 
is obtained by dissolving 1 Ib. of cadmium sulphate in 4 gallons 
of water and adding 1 J gallons of the ordinary yellow ammonium 
sulphide. 3. Or a solution of cadmium sulphate is made, to 
which is added a solution of sodium thiosulphate and a little 
sulphuric acid and the mixture boiled for an hour. This variety 
contains much free sulphur, and is, hence, liable to undergo 
oxidation to sulphuric acid, which destroys the yellow. 

Preparation of Orange Cadmium. 1. A solution of cadmium 
sulphate or chloride is prepared. It is made strongly acid by the 
addition of excess of hydrochloric acid, and a current of sul- 
phuretted hydrogen gas is passed through it. 2. 1 Ib. of cadmium 
sulphate is dissolved in 4 gallons of water; the solution is boiled, 
and while boiling, yellow ammonium sulphide is added. All the 
precipitates of yellow obtained in the various ways just described 
must be well washed in water, especially those obtained with 
the ammonium sulphide, to free them from any trace of acid or 
sulphide which, if left in, would ultimately lead to the destruction 
of the colour. After being washed they should be thoroughly 
dried at as low a temperature as possible, not exceeding about 
150 to 160 F. ; too high a temperature causes the shade to 
become brown while hot and although the colour comes back on 
cooling, yet it never quite regains the original brilliancy. 

PROPERTIES OF CADMIUM YELLOW. Pure cad- 
mium yellow is one of the most permanent pigments known ; it 
is unaffected by exposure to light and air. It mixes with any 
vehicle used in painting. When heated strongly the colour 
darkens, changing to a dark violet-red; on cooliiig, the original 



AUREOLIN. 149 

colour comes back, not, however, always in its original brilliance, 
but with a brownish tone. The impure yellows, those which are 
made with yellow ammonium sulphide or sodium thiosulphate, 
are not permanent pigments. When they are exposed to the 
combined action of air and moisture, the free sulphur they con- 
tain becomes oxidised to sulphuric acid, and this, acting on the 
yellow cadmium, changes it to sulphate, which change is shown 
by a bleaching of the colour, and occurs whether the pigment be 
ground or used in oil or water. 

Cadmium yellow can be mixed with almost all the other pig- 
ments without affecting them or being affected by them ; the only 
exceptions are those pigments which, like white lead, emerald- 
green, and the chrome-yellows, contain lead or copper as their 
basis. When such pigments are mixed with cadmium yellow 
double decomposition sets in, resulting in the formation of black 
sulphide of lead or copper as the case may be ; the production of 
either compounds causes the mixture to acquire a greyish or 
brownish tint. 

ASSAY AND ANALYSIS OP CADMIUM YELLOW. 
Besides the usual tests for colour and body, cadmium yellow 
should satisfy the following tests : Strong hydrochloric acid 
should completely dissolve the yellow with evolution of sul- 
phuretted hydrogen to a clear colourless solution, from which, on 
dilution with water and passing sulphuretted hydrogen gas, a 
yellow precipitate only should be obtained. The nitrate from 
this precipitate should give no precipitates on adding ammonia 
and ammonium sulphide. The addition of barium chloride to 
the solution should produce no turbidity. On boiling with caus- 
tic soda, filtering off the residue and adding hydrochloric acid to 
the filtrate no yellow precipitate, indicating the presence of 
arsenic yellow, should be obtained. Carbon bisulphide should 
extract no sulphur from it. Samples should not yield anything 
to water when boiled with it. The aqueous liquor should not 
give any precipitates with silver nitrate or barium chloride nor 
any acid or alkaline reactions to test papers. 

Cadmium yellow is rarely adulterated ; the common adulter- 
ants are arsenic yellow, zinc chrome, and the chrome-yellows, 
the presence of which can be distinguished by the application of 
the characteristic tests, which are given under the respective 
pigments. 

AUBEOLIN. 

This pigment is a double nitrite of potassium and cobalt pre- 
pared by precipitating cobalt nitrate with sodium carbonate, 



:P 



150 YELLOW AND ORANGE PIGMENTS. 

dissolving the precipitate in acetic acid and adding a strong solu- 
tion of potassium nitrite. On allowing the mixture to stand for 
some time the colour is gradually precipitated, and is collected 
and washed ; after being dried it is ready for use. 

Aureolin is of a bright yellow colour, but is not permanent, 
being affected by exposure to light and air; acids dissolve it, 
while alkalies have no action. 



151 



CHAPTER V. 
GREEN PIGMENTS. 

THE green pigments in common use by the painter and the 
artist are derived from both natural and artificial sources, but 
usually from the latter. The green pigments are valuable, 
largely made and used, and are fairly numerous. 

The commonest are those known as Brunswick greens, which 
are made in very large quantities for common painting ; next is 
emerald-green, although this colour, owing to its poisonous 
nature, is gradually being displaced by substitutes made from the 
coal-tar greens ; then comes the true chrome-green ; then some 
of the other copper-greens ; while the rest are only used on a 
limited scale. 

BRUNSWICK GREEN. 

Under the names of " pale Brunswick green," " middle Bruns- 
wick green," " deep Brunswick green," are sent out several green 
pigments, varying in shade or tint from a pale yellowish-green to 
a very deep blue-green. These pigments are made in very large 
quantities, and are mixtures, in various proportions, of chrome- 
yellow, Prussian blue, and barytes. They must not be con- 
founded with the pigment originally known under the same 
name, which was a compound of copper, and which has almost 
completely gone out of use. 

Brunswick greens can be made in various ways, almost every 
colour maker having his own favourite manner of mixing the 
various ingredients together. 

DRY METHOD. In this method the materials composing 
the green are thrown into the pan of an edge-runner grinding 
mill or into a mixing mill ; the former is preferable, as the 
materials are ground as well as mixed, and this has some 
influence in developing the tint of the green. The main advantage 
in this method is considered to be that the shade of green which 
is being produced is visible while in the mill, and that if too 



152 GREEN PIGMENTS. 

much yellowing is being produced as compared with a standard 
sample, then, by throwing a little more blue into the mill, the 
fault can be remedied at once ; but against this advantage must 
be set off the disadvantage that the tints of green are not so fine 
as those which are obtainable by wet methods. The following 
proportions can be taken as guides in making the various shades 
of Brunswick green by this dry method : 

Pale Brunswick Green. 1 cwt. of barytes, 1J Ibs. of 
Prussian blue, and 35 Ibs. of chrome-yellow. 

Middle Brunswick G-reen. 1 cwt. of barytes, 2J Ibs. of 
Prussian blue, and 35 Ibs. of chrome-yellow. 

Deep Brunswick Green. 1 cwt. of barytes, 5 Ibs. of Prussian 
blue, and 35 Ibs. of chrome-yellow. 

Extra Deep Brunswick Green. 1 cwt. of barytes, 8 Ibs. of 
Prussian blue, and 35 Ibs. of chrome-yellow. 

The quality and tint of the chrome-yellow used will be found 
to have some influence upon the tint of the green produced ; for 
the pale shades lemon-chromes should be used ; while for the 
deep shades the middle shades of chrome can be used. Before 
making up a large batch of green with a new batch of either 
chrome-yellow or Prussian blue a small trial lot should be made 
to see if the two pigments will produce the required shade, as 
experience shows that different makes of chrome-yellows, even if 
of the same shade, do not always give the same results in making 
these greens ; while the difference between two makes of Prussian 
blue as green-producers is greater than that between two makes 
of chrome-yellow. 

For making the palest shades of Brunswick greens, only the 
best and brightest Prussian blues should be used ; for the darker 
shades of green the quality of the blue is not of so much con- 
sequence. 

The barytes may be replaced with gypsum, if thought fit ; less 
gypsum is required to produce a given tint of green than barytes, 
the proportion being about 1 cwt. of gypsum to 21 cwts. of barytes. 

WET METHODS. The wet methods are those commonly 
adopted by makers of Brunswick greens, partly because they are 
the oldest (for colour makers are so conservative that it is 
difficult to induce them to alter their methods), and, partly, 
because the wet methods produce the finest greens; but they are 
very much more troublesome to carry out and require no little 
practical experience on the part of a colour maker to produce the 
best results. From the recipes which have been given above it 
will be seen that barytes is the principal ingredient in these 
greens ; hence the principle which should underlie all wet pro- 



BRUNSWICK GREENS. 153 

cesses is to precipitate the yellow and the blue constituents on 
the barytes simultaneously. This is by no means easy to do, and 
yet much of the brilliancy of the green depends upon this being 
done as successfully as possible. The best way to ensure this 
result would be to mix solutions of the acetates of lead and iron 
or of the nitrates of those metals together and to add the barytes 
and precipitate with a mixture of the bichromate and ferrocyanide 
of potash, but, unfortunately, this course is not at present 
available, for the reason that while the acetate or nitrate of lead 
can be purchased on a commercial scale of sufficient purity, the 
acetate or nitrate of iron is not so purchasable \ the common iron 
liquor is too impure for use in making greens, while the nitrate 
of iron so-called is of too variable a composition to be recommended 
for use in colour making. 

The materials which are used in the preparation of the greens 
are copperas (ferrous sulphate), which should be used as pure 
and as fresh as possible, the variety known as green copperas is 
the one required, acetate of lead, bichromate of potash, ferrocy- 
anide of potassium (yellow prussiate of potash), and barytes. The 
red prussiate of potash would give rather better results than the 
yellow, only that its extra cost is against its use for making 
greens for common use ; but when a good price is obtainable its 
use is to be recommended, as the green is much easier to make 
with it than with the yellow prussiate. 

For producing the various shades of Brunswick greens the 
following proportions may be used which, as well as those given 
above, may be varied so as to suit the special requirements of 
each individual maker. The following points are, however, well 
worth attending to in making alterations in the proportions. 1st. 
That equal weights of prussiate and copperas must be used. 2nd. 
That the proportion of acetate of lead to the bichromate of potash 
should be, as nearly as possible, 10 to 3J. 

Pale Brunswick Green. 1 cwt. of barytes, 13 Ibs. of acetate 
of lead, 1 Ib. of copperas, 1 Ib. of yellow prussiate of potash, and 
4 Ibs. of bichromate of potash. 

Middle Brunswick Green. 1 cwt. of barytes, 13 J Ibs. of 
acetate of lead, 1|- Ibs. of copperas, 1J Ibs. of yellow prussiate of 
potash, and 4 J Ibs. of bichromate of potash. 

Deep Brunswick Green. 1 cwt. of barytes, 14 Ibs. of 
acetate of lead, 2 Ibs. of copperas, 2 Ibs. of yellow prussiate of 
potash, and 4J Ibs. of bichromate of potash. 

Extra Deep Brunswick Green. 1 cwt. of barytes, 16 Ibs. 
of acetate of lead, 4 Ibs. of copperas, 4 Ibs. of yellow prussiate of 
potash, and 5 Ibs. of bichromate of potash. 



154 GREEN PIGMENTS. 

Instead of the bichromate of potash the bichromate of soda 
may be used with some advantage, on account of its greater 
solubility and less cost. 

Instead of barytes, any white pigment may be used, when it 
will be found that a smaller quantity will be required to give a 
green of the same depth of colour ; or, to put it in another way, 
an equal weight of another white base will require a greater 
quantity of the other ingredients to produce the same shade of 
colour ; with gypsum the proportion will be about 2J times as 
much, with china clay about 3 or 4 times as much, and with zinc 
white from 4 to 5 times as much ; it is very rarely that any 
other pigment than barytes is used in making these greens. 

By varying the proportions a great variety of green shades can 
be produced. It is mainly owing to the variable proportions 
used that the middle green, say, of one maker so seldom has 
exactly the same tint or depth of colour as that of another 
maker. 

The following is the best method of working, in order to 
obtain good bright pigments with the ingredients given above. 
The iron salt is dissolved in a tank of cold water, the lead salt is 
similarly dissolved in another tank, while the two potash salts 
can be dissolved together in one tank. The barytes is thoroughly 
mixed with water in another tank, and, when properly mixed, 
the iron solution is run in with constant stirring ; and then the 
lead salt is run in. Some of the lead will be precipitated as the 
sulphate, owing to double decomposition taking place between 
the two salts, but this cannot be avoided, so that allowance 
should be made for it in all recipes for making Brunswick green 
by increasing the amount of acetate of lead as the quantity of 
copperas is increased ; every extra pound of the latter will 
require 1 Ib. 3 oz. of acetate of lead to be added in addition to 
that required to form chrome-yellow with the bichromate. After 
the lead has been run in and mixed with the rest of the in- 
gredients, the whole is kept stirred while the potash salts are 
run in ; the green soon forms and is allowed to settle ; the clear 
liquor at the top is run off and the pigment washed by running 
in water and stirring well, again allowing to settle and running 
off the wash waters ; this washing should be repeated once or 
twice. Then the colour is taken out, thrown on niters to drain, 
and, finally, dried at a gentle heat. Various other methods of 
manipulating the preparations of these greens are in use among 
the various makers, but it is not necessary to describe them. 

A method of working, which has been used by the author with 
very good results as to ease and quality of the colour produced, 



COMPOSITION AND PROPERTIES OF BRUNSWICK GREENS. 



is to grind all the ingredients together, in the proportions given. 
above, in an edge-runner mill, and when they are properly 
mixed, to put them into the tub and run water on to them, with 
constant stirring ; the green is rapidly developed, and is allowed 
to settle ; the clear top liquor is then run off, and fresh water 
run on to wash the pigment, after which it is finished as usual. 
These greens are sold under a variety of names Brunswick 
green (which is the commonest), Chrome-green, Victoria green, 
Prussian green, &c. 



COMPOSITION AND PROPERTIES OF BRUNSWICK 

GREENS. 

These pigments are compounds of barytes, chrome-yellow,. 
Prussian blues, with occasionally small quantities of lead sulphate, 
gypsum, and other bodies. The following analyses made by 
the author of different shades of these greens, of a good make, 
will show the average composition of the pigments : 

ANALYSES OF BRUNSWICK GREENS. 





Pale. 


Middle. 


Deep. 


Extra 












Water, . 


1-000 


0-800 


i-ooo 


0-800 


Barytes, . 


72-744 


71-944 


72-764 


71-404 


Gypsum, . 


2-341 


traces. 


traces. 




Prussian blue, . 


0-929 


0-442 


4543 


6-910 


Chrome-yellow, 


14-266 


25-734 


17-660 


17-867 


Lead sulphate, 


8-073 


1-039 


3-280 


3-152 




99-353 


99-959 


99-247 


100-130 













They are good pigments, and work well both in oil and water, 
especially the former ; their opacity is good, and, therefore, they 
have good body or covering power, in this respect surpassing all 
other green pigments. They can be mixed with other pigments, 
with but few exceptions, without any change being brought 
about by interaction ; these exceptions being those pigments 
containing sulphur, which would act upon the chrome-yellow 
and darken the green, by the production of black lead sulphide 
and highly basic colours, like whiting or lime, which would act 



156 GREEN PIGMENTS. 

both upon the chrome-yellow and the blue, turning the green 
into a red. 

They are fairly permanent when exposed to light and air, for, 
although not quite permanent, they are so for all practical 
purposes ; exposure to light causes the yellow constituent to 
fade first, as a rule, so that, especially in the dark shades, the 
green has a tendency to turn blue, but in this respect the blue 
is very variable ; in some makes the yellow goes first, in others 
the blue, much probably depends upon the composition of the 
particular green and the circumstances under which it is placed. 

Acids turn the colour bluer, owing to their dissolving out the 
chrome-yellow ; on the other hand, alkalies turn it orange, 
owing to their combined action both on the blue (turning this 
of a reddish-brown) and on the yellow (which they turn orange), 
as is noted in describing the blue and the yellow in their 
respective places. Sulphuretted hydrogen darkens the tint 
considerably. 

ASSAY AND ANALYSIS OF BRUNSWICK GREENS. 
Brunswick greens require assaying for colour or tint, covering 
power or body, brilliance, &c., by the usual methods. Since, as 
already stated, the pale shade of one maker may not exactly 
agree with the pale shade of another maker, the different makes 
should always be compared together for the various properties 
just named ; as a rule, it will be found that different batches of 
the same maker's green will match one another very closely. 

It is rarely that an analysis of these greens is required ; but, 
if so, the following method, described by Brown,* may be 
followed : 

1. For Chrome Green. Weigh out 2 grammes of the green, 
treat with 28 to 30 cc. of strong hydrochloric acid, at the boil, for 
about 10 minutes, then filter while still hot, and wash well with 
boiling water, adding the wash waters to the filtrate. 

The residue, consisting of barytes and Prussian blue, is 
strongly heated until the blue is decomposed, and, after cooling, 
the residue is weighed; this gives the weight of oxide of iron 
and barytes in the green. Treat the residue with a mixture of 
nitric and hydrochloric acids, boil well, then dilute with water 
and filter; dry, ignite, and weigh the residue, which is the 
barytes ; deduct this weight from the original weight of barytes 
plus oxide of iron ; the difference is the amount of oxide of iron, 
which, multiplied by 2 '2 12, gives the amount of Prussian blue 
in the green. 

Filtrate. Nearly neutralise by the addition of ammonia, then 
* Brown, Chemical Neivs, December 31, 1886. 



ASSAY AND ANALYSIS OF BRUNSWICK GREENS. 157 

pass a current of sulphuretted hydrogen gas through the solution, 
which will precipitate the lead as lead sulphide ; filter this off 
and wash the precipitate, adding the wash water to the filtrate. 

Lead Sulphide. Treat the precipitate of lead sulphide with hot 
nitric acid, boil down to a small bulk, then add a little strong 
sulphuric acid, heat until acid fumes begin to appear, then allow 
to cool, add water and a little alcohol, filter, wash, and, after 
drying and igniting the precipitate in the usual way, weigh as 
lead sulphate. This gives the total amount of lead in the green, 
which may be in the condition of sulphate as well as of chromate 
(see the analyses given above). 

Filtrate from the Lead Sulphide. This contains the chromium 
and occasionally a little iron. Boil down to a small bulk, then 
test the solution for iron by taking a drop out with a glass rod 
and placing it on a piece of paper moistened with potassium 
ferrocyanide; if a blue spot appears, then iron is present, and the 
solution is treated according to method No. 1 ; if iron is absent 
it is treated according to method No. 2. 

Method No. 1. Boil the solution with nitric acid and potassium 
chlorate until a clear yellow solution is obtained, then add 
sufficient ammonia to precipitate the iron, filter off, wash, dry, 
and weigh the precipitate, which weight is to be added to the 
weight of the iron previously found. Take the filtrate from the 
iron precipitate, boil down to a small bulk, add some strong 
hydrochloric acid and a little alcohol and boil until the colour of 
the solution becomes a clear green ; this is effected by cautious 
addition of more acid and alcohol, if required, but too great 
excess of either must be avoided ; to the solution is added 
ammonia in excess and the mixture is boiled until it gives on 
filtering a colourless filtrate ; the precipitate consists of chromium 
hydroxide, which is filtered off, dried, ignited, and weighed ; the 
weight multiplied by 4-241 gives the weight of chrome-yellow 
(pure chromate of lead) in the green. 

Method No. 2. The filtrate from the lead sulphide in which no 
iron is present is boiled and ammonia added in excess ; the mix- 
ture is then treated as described under method No. 1, from the 
same point. 

2. For Lead Sulphate. Weigh out 2 grammes of the green, 
boil with hydrochloric acid, filter while still hot, and wash with 
boiling water ; evaporate the filtrate down and, while still boiling, 
add barium chloride in slight excess ; filter, wash well with 
boiling water, and treat the precipitate of barium sulphate in the 
usual way. The weight of barium sulphate multiplied by 1*3 
gives the weight of lead sulphate in the green. The difference 



158 GREEN PIGMENTS. 

between this amount and that found in the first instance repre- 
sents the lead which is present in the green in other forms. 

A qualitative analysis should always precede a quantitative 
one ; and the above scheme should be modified if such an 
analysis shows that other constituents, besides those mentioned 
above, are present in the greens. The notes on the analysis of 
chrome-yellows (p. 127) will be found useful. 

BRUNSWICK GREEN. 

The modern Brunswick greens must not be confused with the 
pigment which was made and sold at one time under this name 
and which has now become quite obsolete. This old Brunswick 
green was a basic chloride of copper, sometimes called an oxy- 
chloride, Cu 2 O C1 2 . It can be prepared by several methods. 

1. 20 Ibs. of copper turnings are placed in a vessel capable of 
being closed ; over them is poured a solution of 30 Ibs. of 
ammonium chloride in 6 gallons of water ; the vessel is closed up 
and the contents well mixed by shaking ; the vessel is kept in a 
warm place for about two months, and at intervals the contents 
are mixed by shaking up ; at the end of the time the vessel is 
opened, when it will be found that most of the copper has been 
converted into the green oxy chloride. 

2. Into a wooden tub is placed about 1 cwt. of old sheet 
copper cut into small pieces ; over them is poured a solution of 
105 Ibs. of sulphate of potash and 1'cwt. of common salt; the 
mass is allowed to react together for some time, the length of 
which depends upon the temperature, and is longer in winter 
than in summer. The green gradually forms, and when it is 
seen that most of the copper has been converted, the green is 
separated from the undecom posed copper by sieving and washing. 

3. 1 cwt. of copper is mixed with 67 Ibs. of salt and 34 Ibs. of 
sulphuric acid mixed with 3 times its volume of water; after 
standing some time the green is formed, when it is treated as 
before. 

4. Metallic copper is taken and just covered with a strong 
solution of chloride of copper and left until it is changed into the 
basic chloride, when it is finished as described under method 2. 

The preparation of Brunswick green is a very slow operation, 
extending over 2 to 4 months as a rule ; in all cases the green is 
collected by washing it with water to free it from any alkaline 
bodies, sieving to free it from unchanged copper, drying slowly 
at a low temperature, since high temperatures tend to decompose 
it ; necessarily it is somewhat costly. 



CHROME-GREEN. 159 

As a pigment it is fairly good, working well both in oil and 
water, and having a good covering power ; in tint it has some- 
what of a bluish-green cast of no great depth of colour. It is not 
quite permanent, although it resists some considerable amount 
of exposure to light and air. In its general properties it closely 
resembles the Bremen blues and the Bremen greens, which see. 



CHROME-GREEN. 

True chrome-green is a most valuable pigment, not only on 
account of the brilliance of its colour, but also on account of its 
great permanence, being in fact the most permanent green 
pigment known. 

In its chemical composition chrome-green varies somewhat 
according to the method of making; in some cases it consists 
entirely of the oxide of chrome, Cr 2 3 ; in others of the phosphate 
of chrome, O 2 2 P O 4 ; while in yet others it is a mixture of these 
bodies. It should not be confused with the mixture of chrome- 
yellow and Prussian blue which is sometimes sold as chrome- 
green. 

Various methods are in use for preparing chrome-green. 

1. Guignet's Process. Guignet was one of the first to prepare 
chrome-green, if not the very first; hence the pigment is 
frequently sold under the name of " Guignet's green." Guignet 
uses boric acid (boracic acid) and bichromate of potash. As a 
rule, the commercial articles are of sufficient purity to prepare 
good pigments with. But if very good results are required it is 
advisable to purify them by re-crystallisation. 

88 Ibs. of potassium bichromate and 33 Ibs. of boracic acid are 
ground into a stiff paste with water; the mixture is then put 
into a furnace where it is heated to a dark red heat for 4 hours. 
A form of reverberatory furnace is the best that can be used. 
The fused mass is thrown into water and repeatedly washed by 
decantation ; the washed pigment is ground whilst still wet 
under an edge-runner mill, again washed, filtered, and dried. 

The first wash-waters contain a good deal of the boracic acid 
in the form of potassium borate ; this acid may be recovered and 
used over again ; the waters are boiled down a little and to the 
liquor is added hydrochloric acid; this throws out the boracic 
acid, which gradually collects in the form of crystals on standing; 
these crystals can be collected and used for making another 
batch of green ; in this way at least 70 to 75 per cent, of the 
boracic acid originally used is recovered. 



160 GREEN PIGMENTS. 

The reaction which takes place between the boracic acid and 
the bichromate is expressed in the following equation : 

3K 2 2 7 + 2H 3 B0 3 = 3Cr 2 3 + 2K 3 B0 3 + 3H 2 + 90 

from which it can be calculated that a considerable excess of 
boracic acid has been used in the process of making the colour ; 
this excess is not wasted, since part is recovered, and, moreover, 
an excess is necessary for the production of a fine quality of the 
pigment. Borax cannot be substituted for the boracic acid. 
Chrome-green made by this process has a fine yellow-green tint. 

2. 3 Ibs. of bichromate of potash and 2 Ibs. of ammonium 
chloride are thoroughly mixed together into a paste with water ; 
this is dried and then calcined at a red heat in a furnace ; the 
calcined mass is well washed in water, and the pigment thus 
obtained is ground. This process gives a fine quality of green, 
but it is not quite equal to that obtained by the last process. 

3. When solutions of ammonia or caustic soda or carbonate of 
soda are added to solutions of the basic chromium salts a pre- 
cipitate of the hydroxide, Cr 2 H G O 6 , is obtained ; when this is 
heated to redness it loses water and passes into the oxide, Cr 2 (X; 
the tint of green obtained in this way is not good, being of a 
greyish hue; by mixing with the precipitate some salt before 
calcining, and afterwards thoroughly washing with water, the 
tint of the green is materially improved. 

All the above processes yield the green in the form of oxide, 
Cr 2 O 3 , which is perfectly permanent when used as a pigment. 

4. A solution of chromium chloride is prepared by heating a 
strong solution of potassium bichromate with hydrochloric acid 
and a little methylated spirit ; to this solution is added, first, 
sufficient soda to neutralise the acid, and, then, a solution of 
sodium phosphate ; the precipitate of chromium phosphate is 
collected, dried, and calcined; the green is then finished by 
washing and grinding in the usual way. 

5. A cheaper method of producing the phosphate consists in 
preparing a solution of 10 Ibs. of potassium bichromate and 
18 Ibs. of sodium phosphate; the mixture is boiled, and, while 
boiling, a solution of 10 Ibs. of sodium thiosulphate is added, and 
then a little hydrochloric acid. On continuing the boiling the 
chromium phosphate is slowly precipitated; when the precipi- 
tation is complete, the green is treated as in the last process. 
The pigment obtained by this process is apt to contain a trace of 
sulphur, which introduces into it an element of change. 

The phosphate-of-chrome greens are by no means equal to the 
oxide-of-chrome greens for brilliancy of tint. 



COPPER GREENS. 161 

PROPERTIES OF CHROME -GREEN. Chrome-green 
forms a fine green pigment of a slightly yellowish tone ; it mixes 
well with either oil or water, has good body or covering power, 
and is quite permanent, being one of the best pigments which 
the painter can use, on which account it is much used by artists. 
It mixes with all other pigments without being affected by them 
or altering them in any way. 

When properly made it is quite insoluble in either acids or 
alkalies. The solubility of oxide of chromium depends upon the 
temperature and length of time to which it has been heated; the 
greater these two factors are the more insoluble becomes the 
oxide, so that well prepared oxides are very insoluble owing to 
the fact that they have to be heated to a high temperature for 
some time. 

Very nearly the same property is found in the phosphate 
greens. 

ASSAY AND ANALYSIS OF CHROME-GREENS. 
Chrome-greens should be assayed for colour, brilliance, covering 
power, and similar properties in the usual way. When pure, 
chrome-green should not impart a yellow colour to dilute 
hydrochloric acid when boiled with that reagent, such yellow 
colour would indicate adulteration with chrome-yellow. When 
boiled with caustic soda chrome-green should remain unacted 
upon. The liquor should be divided into two portions to the 
one acetic acid should be added, when no yellow precipitate 
indicating chrome-yellow should be obtained ; to the other 
hydrochloric acid and ferric chloride should be added, when no 
blue precipitate should be obtained, such precipitate would 
indicate the presence of Prussian blue. Chrome-greens are 
usually adulterated with the Brunswick greens, which adultera- 
tion is detected by the application of the two tests just given. 
For use by calico-printers, Guignet's green is supplied in the 
form of a paste, containing usually 30 per cent, of actual colour. 

COPPER GREENS. 

Copper forms the base of a number of greens, some of which 
are of value, although the bulk are of but minor importance, and 
their use is gradually decreasing. The variety of names under 
which the copper-greens have been offered from time to time is 
very great ; very few are now in use, and it is rather difficult to 
know exactly to what copper compound any particular name one 
meets with in old books and papers belongs. One of these 
greens has already been described. 



162 GREEN PIGMENTS. 

Three of these copper-greens verdigris, Scheele's green, and 
emerald green are very closely related to one another, and form, 
as it were, a group of colours. Verdigris is the basic acetate of 
copper. Scheele's green is the arseniate of copper, while emerald 
green is the aceto-arsenite of copper, and may be viewed as a 
compound of the other two colours. 



VERDIGRIS. 

The chemical composition of verdigris has been already stated. 
It is made in two forms, known as "distilled" and "common" 
verdigris. The first, being somewhat of a crystalline nature, is 
rarely used as a pigment, and finds its chief use in medicine ; 
the latter is of the most importance from a painter's point of 
view. 

Preparation of Verdigris. Distilled verdigris is prepared 
by dissolving copper or oxide of copper in the acid obtained 
during the distillation of wood, from which circumstance arises 
the name "distilled verdigris;" for the product itself is not 
distilled. Another method of manufacturing this variety is to 
mix together solutions of sulphate of copper and acetate of lime, 
or of the acetate of lead ; the sulphate of lime or lead, as the 
case may be, is precipitated, and a solution of acetate of copper 
is obtained. From the solution of acetate of copper, obtained by 
either of the above methods, the verdigris is obtained by con- 
centrating down to the crystallising point, and allowing the salt 
to crystallise out ; this gives the best product. Or, the solution 
may be cautiously evaporated to dryness; this is costly and 
there is a risk of decomposing the green, causing it to lose its 
brilliancy of tint. 

Distilled verdigris occurs in the form of dark green crystals, 
soluble in water and in acetic acid ; as a pigment it is of little 
use, being too transparent; then, again, its solubility is against 
its being a good pigment. 

Common verdigris is prepared in several ways. 

1. French Process. The skins and marc of grapes, left after 
the juice has been pressed out for making wine, are used in 
France for making verdigris; the material is placed in large 
tubs, loosely covered over with netting, in which it remains for 
a few days, when acetic fermentation sets in ; when this has 
commenced sheets of copper (averaging about 8 inches by 4 inches) 
are thrown in among the fermenting mass generally old scrap 
copper is used. They are left in the tub among the grape skins for 
from 18 to 20 days, the period varying according to the weather; 



COMPOSITION AND PROPERTIES OF VERDIGRIS. 163 

in summer it may be only about 12 to 14 days, but in winter the 
longer period named is always required. At the end of the 
time the tubs are emptied and the grape refuse thrown away ; 
the copper sheets are dried, then dipped into water, or, what is 
better, into bad wine (if that is obtainable), and again dried ; by 
this means a coat of verdigris is formed on the plates, which 
is scraped off and placed on one side ; the plates are redipped 
and again dried, when another coating of verdigris is formed, 
and scraped off as before ; the process is repeated until all 
the copper has been converted into verdigris. The green is 
washed with water and then dried, when it is ready for use. 
At one time almost every vineyard in France and Belgium 
made verdigris somewhat on the above lines, although there 
were some variations in the minor details ; but, as the consump- 
tion of verdigris has decreased considerably, its manufacture has 
not been so generally followed of late years. 

2. English Process. In England verdigris is made by packing 
plates of copper between cloths soaked in the crude pyroligneous 
acid obtained in the distillation of wood ; this is done in casks ; 
every four or five days the casks are unpacked and the cloths 
redipped in the acid, and the operation repeated until the sheets 
of copper begin to have a coat of verdigris ; they are then dipped 
in water and dried ; the verdigris on them is then scraped off 
and the copper is again packed with the cloths ; and the process 
repeated until all the copper has been converted into verdigris. 
The refuse from the manufacture of cider has been used in 
making this pigment. 

The verdigris is finished for use by washing and drying. The 
latter has to be done very carefully, as too high a temperature 
would affect the brilliancy of the tint. 

Common verdigris is not quite so pure as distilled verdigris ; 
but as it is more insoluble in water and more opaque, it can be used 
as a pigment. 

COMPOSITION AND PROPERTIES OF VERDIGRIS. 
Phillips gives the following analysis of distilled verdigris : 

Copper oxide, CuO, . . . . 43 '25 percent. 
A cetic anhydride, C 2 H 3 . O . C 2 H 3 O, 28 '30 
Water, H 2 0, 28 45 

which corresponds to the formula 

Cu 2 C 2 H 3 2 , Cu H 2 2 , 5 H 2 0. 

Distilled verdigris is very constant in its composition. It 
forms dark green crystals somewhat soluble in water and in 



164 GREEN PIGMENTS. 

acetic acid ; heated in the air they lose their water of crystallis- 
ation and their acetic acid, and a black residue of copper oxide, 
Cu O, is left behind. Common verdigris is very variable in its 
composition and usually contains some impurities ; as a rule, it 
is generally allowable for commercial verdigris to contain about 
2 per cent. 

The following analyses of some samples of common verdigris 
will show the average composition of this pigment : 

1. 2. 3. 4. 5. 

Copper oxide, 44-25 4379 40 '79 43 "24 43 '5 

Acetic anhydride, 29 '62 38 '49 45 '97 27 '57 29 "3 

Water, . . 23'51 18*00 13'04 29'19 25-2 

Impurities, . 2'62 ... ... ... 2'0 

1 is by Phillips of an English-made sample. 2 and 3 are by the author ; 
the water in these was present both as hygroscopic and combined water in 
No. 2 the amounts were respectively 7 '95 and 10 '05, in No. 3 they were 
4'49 and 8'75. 4 and 5 are by Berzelius of French-made samples. 

Verdigris has a greenish-blue colour. It makes but a poor 
pigment, being the most fugitive of the copper-greens ; in water 
it soon fades, in oil it is rather more permanent, if kept free from 
moisture, which causes it to effloresce. 

While being almost insoluble in water, it is readily soluble in 
all acids, without effervescence, to a blue solution, which gives the 
characteristic tests for copper. Heated with strong sulphuric 
acid it evolves acetic acid. Heated alone it loses its water 
and acid and turns black, from the production of oxide of 
copper. 

ASSAY AND ANALYSIS OF VERDIGRIS. Verdigris 
may be assayed for tint, covering power, &c., in the usual 
way. 

The following impurities in verdigris should be tested for; 
Insoluble matter, carbonates, sulphates, and metals : 

Insoluble Matter, which may consist of barytes, sand, &c., is 
readily tested for. A weighed quantity is treated with hot 
dilute hydrochloric acid for a short time and the insoluble matter 
collected on a filter, dried, and weighed. The amount of in- 
soluble matter should not exceed 3 per cent., and even this is 
an excessive allowance, as can be inferred from the analyses 
already quoted ; 2 per cent, is, in the opinion of the author, quite 
a sufficient allowance, and anything above this ought to be 
considered as an adulteration. 

If, when treating with the acid, effervescence occurs it may be 
taken as an indication of the presence of carbonates either of 
copper or calcium, or both. 



165 

Sulphates can be tested for in the hydrochloric acid solution 
by adding barium chloride, and their amount may be determined 
by filtering off, drying, and weighing the precipitate of barium 
sulphate, which will be formed if they are present. 

The presence of other metals tJian copper is best ascertained by 
working according to the following scheme. The pigment is 
heated to a red heat to decompose it, and the residue is dissolved 
in hydrochloric acid and any insoluble matter filtered off; 
through the solution a current of sulphuretted hydrogen is 
passed, to precipitate out the copper as the black sulphide ; the 
filtrate is boiled down to a small bulk, a little strong nitric acid 
is added, and the mixture is boiled for a few minutes ; ammonia 
is then added in slight excess ; if any iron is present a reddish- 
brown precipitate will be obtained ; this is filtered off and to the 
filtrate ammonium sulphide is added, which, if zinc is present, 
will throw down a white precipitate of the sulphide of zinc; 
after filtering this off, ammonium oxalate is added to the filtrate 
when, if calcium is present, a white precipitate of calcium oxalate 
will be obtained. Calcium may be present either in the form of 
carbonate or sulphate, which will be inferred from the results of 
other tests. It is rare for any other metal to be present. 

Adulteration of verdigris with Prussian blue can be detected 
by the blue residue left after the treatment with acid giving all 
the reactions of Prussian blue. The addition of ultramarine is 
readily detected by the action of acid on the sample. 

Green verditer is sometimes sold under the name of " British 
verdigris." 

SCHEELE'S GREEN. 

This pigment was discovered by Scheele, the eminent Swedish 
chemist, who communicated his method of making it to the 
Academy of Sciences of Stockholm in 1778. At first it was very 
much used, being at that time one of the best greens known, but 
the introduction of emerald-green in 1814 soon brought about its 
gradual disuse, and now it is doubtful whether it is ever used as 
a pigment ; this is partly due to the fact that it is but a dull 
colour, while much brighter and better greens are now known ; 
then, again, the fact of its being an arsenical colour has always 
been much against its use as a pigment. 

PREPARATION OF SCHEELE'S GREEN. 1. Scheele 
gave the following instructions for preparing this green : 1 part 
of powdered white arsenic (arsenious oxide) and 2 parts of potash 
(carbonate of potassium) are dissolved, by boiling, in 35 parts of 



166 GREEN PIGMENTS. 

water ; the solution is filtered and then poured into a solution of 
2 parts of copper sulphate as long as a precipitate falls. The 
precipitate is collected on a filter, washed with water, and dried 
at a gentle heat. 

2. Parker patented, in 1812, a process of making Scheele's 

freen Two solutions were made in boiling water, one containing 
6 ozs. of sulphate of copper, the other 14 drms. of arsenic and 
14 ozs. of potash. The precipitate obtained on mixing the solu- 
tions was washed and dried. As the alkali is greatly in excess 
in this process, the precipitate must consist largely of carbonate 
of copper. 

3. Sharpies* prepares Scheele's green by dissolving 2 parts of 
arsenious oxide (white arsenic) in 8 parts of soda crystals by 
boiling with 10 parts of water ; when dissolved, the arsenite of 
soda formed is poured into a solution of 6 parts of copper sul- 
phate in 40 parts of water. Both solutions are mixed while 
boiling, and the mixture itself boiled for a few minutes ; it is 
then allowed to stand until the next day, when the green super- 
natant liquor is poured off and the green washed two or three 
times with hot water, dried, and filtered. This is stated to be the 
most economical process of making the green. 

4. JSerzelius describes a process of preparing a green by boiling 
copper carbonate with white arsenic ; the green has a fine tint. 

COMPOSITION AND PROPERTIES OF SCHEELE'S 
GREEN. Scheele's green is essentially an arsenite of copper. 
Sharpies, who has made a very exhaustive examination of this 
pigment, states that it is a basic arsenite of copper, usually con- 
taining small traces of carbonate and sulphate of copper. He 
gives the following as the composition of a pure Scheele's green : 

Copper oxide, Cu 0, . . . 50 '00 per cent. 
Arsenious oxide, As 2 63, . . 4200 ,, 
Water, H 2 0, .... 8 00 

which corresponds to the formula 

CuAs0 3 Cu0.2H 2 0. 

The pigment as prepared on a commercial scale differs some- 
what from this, as might be expected ; but the variation is not 
very great when properly made. The following analyses given 
by Sharpies of samples made by Scheele's and Berzelius' pro- 
cesses will show the average composition of this pigment : 

* Sharpies, Chemical News, vol. 35, p. 89 et seq. 



EMERALD-GREEN. 



167 





Berzelius' 
Process. 


Scheele's Process. 




1. 


2. 


3. 


Copper oxide, Cu 0, . 
Arsenious oxide, As2 63, 
Sulphur trioxide, S Os, 
Water, H 2 0, 
Carbonic acid, C 02, . 


66-02 
8-32 

l6'-33 
15-26 


50-76 
40-82 
1-63 
6-41 


49-25 
42-66 
0-42 
6-71 


99-93 


99-62 

. 


99-04 



Sample 2 was made according to the original directions ; but 
sample 3 was washed until the wash-waters were free from 
arsenic. 

Scheele's green is of a pale yellowish-green colour, but not 
very bright ; it is quite insoluble in water, but soluble in dilute 
acids, in dilute solutions of the caustic alkalies, and in ammonia 
with a blue colour; when boiled with solutions either of the 
caustic alkalies or of their carbonates it is decomposed, black 
oxide of copper being deposited ; boiling with ammonia does not 
decompose it. When heated it decomposes, a residue of black 
oxide of copper being left behind and the arsenic being volatilised. 

As a pigment it is not satisfactory; its covering power is 
small, although it can be used either for oil- or water-colours ; it 
is not permanent and fades on exposure to light and air ; in this 
respect it is rather better than any of the other copper-greens 
previously described. As a pigment it has gone out of use. 



EMERALD-GREEN. 

Emerald-green was discovered in 1814, but by whom has not 
been recorded ; from the place at which it was supposed to have 
been first made it is also known as " Schweinfurth green " ; 
while in America it is largely known as " Paris green," under 
which name it is mostly consumed as an insecticide on fruit 
farms. Owing to the brilliancy of its tint, the ease with which 
it works, and its comparative permanence, it has been extensively 
used as a pigment. In composition it is an aceto-arsenite of 
copper, and may be regarded as a compound of verdigris and 
Scheele's green. 

PREPARATION OF EMERALD- GREEN. 1. 125 Ibs. 
of copper sulphate are dissolved in boiling water ; 50 Ibs. of white 
arsenic (arsenious oxide) are boiled with a solution of 130 Ibs. of 



168 GREEN PIGMENTS. 

soda crystals until the arsenic is dissolved ; this solution while 
still hot is poured into the copper solution, when a precipitate of 
copper arsenite will be obtained, and a little carbonate of copper 
also thrown down; sufficient acetic acid is now added to neutralise 
all the carbonate and leave a little in excess; the mixture is now 
allowed to stand for some time for the emerald-green to fully 
develop ; in summer this may take from a week to ten days, in 
winter it will take about three or four weeks ; when formed the 
green is filtered off, washed, and dried. 

2. 8 Ibs. of white arsenic is thoroughly mixed with water and 
then 8 Ibs. of verdigris is stirred in ; on standing for some time 
in a warm place emerald-green begins to form ; when fully 
developed it is filtered, washed, and dried. 

3. 50 Ibs. of copper sulphate are dissolved in water, and to the 
solution is added 10 Ibs. of lime dissolved in 20 gallons of vinegar ; 
to the mixture is added 50 Ibs. of white arsenic previously mixed 
into a paste with water ; the mass is allowed to stand in a warm 
place until the emerald-green has formed, when it is finished as 
above. 

4. Galloway's Process. In the course of an article on emerald- 
green in the. Journal of Science, a few years ago, Prof. Galloway 
described a process for the preparation of emerald-green on rather 
more scientific lines than either of the above processes, and which 
gives very good results. This process is carried out in the 
following manner: A quantity (100 Ibs.) of copper sulphate is 
dissolved in water, and sufficient sodium carbonate (28J Ibs. of 
soda crystals or 12J Ibs. of crystal carbonate) is added to pre- 
cipitate one-fourth of the copper sulphate used in the form of 
copper carbonate ; then acetic acid is added in sufficient quantity 
to dissolve this copper carbonate. There is thus obtained a 
solution containing copper acetate and copper sulphate in about 
the proportions 3 Cu S O 4 + Cu 2 C 2 H 3 O 2 . The copper sulphate 
has now to be converted into copper arsenite ; to do this the 
requisite amount of arsenic (60 Ibs.) is dissolved by boiling in 
sodium carbonate (38 Ibs. of crystal carbonate or 87 Ibs. of soda 
crystals), which is rather less than is required to completely 
precipitate the copper sulphate in the first solution ; the two 
solutions are heated to the boil and then the arsenic solution is 
run into the copper solution ; the green is formed immediately 
and only requires filtering, washing, and drying, for use as a pig- 
ment. The quantities given above have been added by the author, 
and are not given in the original instructions. When carefully 
carried out this process gives excellent results. 

The fineness of the pigment can be regulated by altering the 



EMERALD-GREEN. 169 

strength of the solutions used ; the weaker these are the finer is 
the precipitate and the more beautiful is the tint of the green 
produced. If during the precipitation of the green any tendency 
to form the yellow-green arsenite be noticed, the addition of the 
arsenic solution is stopped, and the mixture is boiled until all the 
yellow-green arsenite is converted into the blue-green emerald- 
green. 

o. M. Camille Koechlin, in 1886, described in the Bulletin of 
the Industrial Society of Mulhouse, a process for the preparation 
of emerald-green. 100 grammes of copper sulphate are dissolved 
in 500 cc. of water; to this is added 187J cc. of a solution of 
arsenite of soda; this solution contains 500 grammes of the 
salt in 1 litre of water. A precipitate of arsenite of copper is 
obtained, and is treated for one hour at from 104 to 122 F. 
with either 62 cc. of acetic acid of 11 to 12 Tw. or 31 cc. of 
pure formic acid ; in either case a fine emerald-green is obtained. 
By using only half the quantity of formic acid a fine blue is 
obtained, a result which is not got with acetic acid. 

6. Liebig's Process. 1 part of verdigris is dissolved by heat in 
acetic acid, then 1 part of arsenious acid, mixed with water, is 
added, and a yellow-green precipitate is obtained. The mixture 
is boiled for some time, and the green gradually forms ; if 
necessary, a little acetic acid should be added from time to time 
to ensure that all the arsenite is converted into the aceto-arsenite ; 
too great an excess of acid, however, should be avoided, as it 
would decrease the yield of emerald-green. As soon as the 
green is fully developed it is filtered off, washed, and dried. 

The drying of emerald-green must be done at as low a tem- 
perature as possible, as heat causes the tint to deteriorate. 

Emerald-green is by no means a difficult colour to make ; the 
first process described takes some time, but the last three are 
quick processes and give good results. 

COMPOSITION AND PROPERTIES OF EMERALD- 
GREEN. Emerald-green is an aceto-arsenite of copper of 
somewhat variable composition, according to the process by 
which it has been made. The following analysis of a sample of 
English-made emerald-green will serve to show the average 
composition of this pigment : 

Copper oxide, CuO, . . . .32*55 per cent. 
Arsenious oxide, As 2 O 3 , . . . 57 '51 ,, 
Acetic anhydride, 2 C 2 H 3 O 2 , . 6 '63 ,, 

Sulphur trioxide, S 3 , . . . T67 ,, 
Water, H 2 O, 0'90 

99-26 

.or THE 



170 GREEN PIGMENTS. 

Leaving out of consideration the impurities, the formula for 
emerald-green deducible from the above analysis is 

7Cu2C 2 H 3 2 , 3CuAs 2 4 . 

Emerald-green is a bluish-green of a very fine tint, quite 
different from any other known pigment, and very difficult to 
imitate ; it is very opaque, and hence has good covering power ; 
it works well in both oil and water, but best in the latter ; kept 
in a dry place it is fairly permanent, and resists exposure to 
light and air, but in a damp place it turns brownish. 

It is soluble in acids to blue solutions ; in ammonia it also 
dissolves with the characteristic copper-ammonia colour; in 
solutions of caustic soda and potash it is also soluble ; on boiling, 
a red precipitate of cupreous oxide falls down, a characteristic 
reaction of emerald-green. 

Emerald-green cannot be mixed with pigments such as 
cadmium-yellow, ultramarine, &c. which contain sulphur, as 
this causes its discolouration, owing to the formation of black 
copper sulphide. With other pigments it can be mixed without 
any alteration. The use of emerald-green has been on the 
decrease of late years, partly owing to its poisonous character, 
due to its containing arsenic, although one authority states that 
there is no foundation for the statement that emerald-green is 
poisonous, and says that it has no poisonous properties whatever. 
The accounts of the poisonous action of emerald-green are very 
conflicting; some persons are much affected by emerald-green; 
even going into a room covered with paper printed with this 
pigment is sufficient to produce poisonous symptoms in them, 
while others are not affected at all ; arsenic seems to be very 
peculiar in its toxic action, and much depends upon the 
physiological idiosyncrasies of the person. 

ASSAY AND ANALYSIS OF EMERALD- GREEN. 
The colour or tint, body, and colouring power of emerald-green 
should be assayed for in the usual way. 

To test for the purity or otherwise of a sample of emerald- 
green the following tests can be applied : In hydrochloric acid 
it should completely dissolve with a yellow-green colour, and on 
diluting with water this colour should turn bluish. It dissolves 
in ammonia with a deep blue colour. In caustic soda it dissolves 
to a pale blue solution, from Which, on boiling, a red precipitate 
of cupreous oxide falls down. The solution in hydrochloric acid 
should not give more than a faint precipitate with barium 
chloride, showing the absence of sulphates ; through the solution 
a current of sulphuretted hydrogen should be passed for some 



MINERAL GREEN. 171 

time and the precipitate of copper and arsenic sulphides obtained 
filtered off; the filtrate after boiling, to free it from excess of 
sulphuretted hydrogen, should give no further precipitate on the 
successive addition of ammonia, amonium sulphat^ and am- 
monium oxalate, showing the absence of metals, such as iron, 
zinc, and calcium. 

The presence of arsenic in emerald-green or other pigments is 
best detected by Marsh's test. This is carried out as follows : 
Provide a wide-mouthed bottle and fit it with a tight-fitting cork 
through which a piece of glass tube drawn out to a point is 
passed. Into the bottle, water, zinc, and sulphuric acid are 
placed. It is necessary that the two latter bodies be free from 
arsenic, as the ordinary commercial articles are very liable to 
contain arsenic which would interfere with the proper testing 
of any pigment for arsenic. By the action of the acid on the zinc 
hydrogen is evolved ; this may be lighted as it issues from the 
glass jet, and will burn with k non-luminous flame. On pressing 
a piece of white porcelain clown on the flame no brownish-black 
spot should be produced. The gas must not be lighted im- 
mediately it begins to issue from the jet, but a few minutes should 
be allowed to elapse before doing so, to allow the air in the bottle 
to be completely driven out ; otherwise an explosion may ensue. 
If, after it has been proved that the gas flame produces no spot 
on a porcelain plate, the sample to be tested for arsenic be 
introduced into the bottle and the gas re-lighted, it will now be 
found to burn with a faintly luminous flame and will give a 
blackish-brown metallic looking spot on a piece of white porcelain 
pressed down on the flame ; this stain is soluble in a solution of 
bleaching powder. Very small traces of arsenic can be detected 
by this test. Another test for arsenic is Reinsch's, which consists 
in heating the sample with hydrochloric acid and a clean copper 
plate ; if arsenic is present the latter becomes covered with a 
grey deposit. 

MINERAL GREEN. 

Under this name and that of Mountain green is offered for use 
as a pigment the natural green mineral known as Malachite. 
There is also an artificial green pigment made under the name of 
mineral green. 

Mineral green, Mountain green, Malachite is a natural basic 
carbonate of copper found in many places Cornwall, Siberia, 
Persia, Australia, &c. The ordinary commercial product comes 
from Siberia where it is found in the greatest abundance and in 



172 GEEEN PIGMENTS. 

large masses of a very fine colour. For use as a pigment the 
natural mineral is simply ground as finely as possible. 

The mineral exists in two forms in the one it forms compact 
masses of a fine yellowish-green tint ; in the other form it is 
rather paler and more porous or powdery in character ; the 
former is the most valuable as a pigment. 

In composition it is a basic carbonate, containing on the 
average 

Copper oxide, Cu 0, . . 71 "9 per cent. 

Carbonic acid, C O 2 , . . . 20'0 
Water, H 2 0, .... 8'1 

lOO'O 

and having the formula Cu C O 3 . Cu H 2 O 2 . 

It makes a good pigment, is fairly permanent, and works well 
both in oil and water. Its faults are those common to all copper 
pigments. 

It dissolves in acids with effervescence and evolution of car- 
bonic acid, and the solution gives all the characteristic tests for 
copper. 

Mineral green has been made artificially by several processes, 
of which the following are two examples : 

(a) For preparing mineral green by this method the following 
materials are required : 2 cwts. of soda crystals, H cwts. of blue 
stone (copper sulphate), 70 Ibs. of quicklime, 12$ Ibs. of white 
arsenic, and 4 ozs. of tartaric acid. Boil the arsenic and soda 
together until the former is dissolved ; dissolve the copper in 
water ; and slake the lime in water. Add the lime to the copper 
solution ; then the arsenic and soda ; and, finally, the tartaric 
acid. Keep the whole at a temperature of about 150 to 160 F. 
for some time until the colour is properly developed, then wash 
the pigment with clean water, filter and dry at a low temperature. 

(b) 14 ozs. of potash and 14 drrns. of arsenic are dissolved in 
water; 1 Ib. of sulphate of copper is dissolved separately in 
2 gallons of water, and into this solution is poured the arsenic 
and potash solution; the green is precipitated and is collected 
and finished in the usual way. 

No process of making mineral green artificially produces it of 
the same deep green tint as the natural variety ; such prepared 
greens have a pale yellowish-green tint and are not so permanent 
as the natural variety. In their general features they resemble 
the verditer greens, but are rather brighter in tint and deeper in 
colour. 

GREEN VERDITER. This pigment is a basic carbonate 



TERRE VERTE. 173 

of copper, prepared by precipitating solutions of copper with the 
carbonates of potash or soda in the same way as blue verditer 
is made. It has a pale tint of a somewhat yellow tone of green. 
As a pigment it has no great use, and has become nearly 
obsolete ; it is not permanent, either as an oil or a water colour. 

Green verditer has also been known as "British verdigris," 
and its preparation under this name has been patented. 

BREMEN GREEN. This is of a pale green tint, prepared 
in the same way as Bremen blue, except that the final blueing 
is omitted. Like green verditer it is essentially a basic car- 
bonate of copper. Its use as a pigment has become obsolete. 

PROPERTIES OP THE COPPER GREENS. With one 
or two exceptions, the copper greens are by no means satis- 
factory pigments; their colour is but pale and not brilliant, 
except emerald-green and the natural mineral green. Their 
covering power is also deficient; they mix with either oil or 
water. They are not permanent, as exposure to light and air 
causes them to fade, while sulphuretted hydrogen and sulphur 
compounds cause them to go black, owing to the formation of 
the black sulphide of copper ; hence they cannot be mixed with 
any pigments containing sulphur such as cadmium yellow, 
King's yellow, ultramarine, &c. Strongly alkaline bodies, such 
as lime, change their colour to a blue. 

Heat decomposes all the copper greens, the acid portion 
carbonic, acetic or arsenious being volatilised, and a black 
residue of oxide of copper being left behind. 

They are all soluble in acids, some with effervescence, indicat- 
ing the presence of a carbonate; the solutions have a blue colour, 
to which ammonia imparts a characteristic deep blue tint ; on 
adding caustic soda a blue precipitate is obtained, which, on 
boiling, turns black. Sulphuretted hydrogen gas passed through 
the solutions throws down a black precipitate of the sulphide, 
which is soluble in nitric acid to a blue solution. 

TERRE VERTE. 

Terre verte is the name given to green pigments of an earthy 
character, found naturally in various places ; in some cases, the 
pigment has been named after the place where it was found, as 
" Yerona green," " Verona earth," &c. These natural greens are 
usually of a pale greyish-green tint, and are only useful on account 
of their permanence. Deposits of green earth are found in many 
places, but only the deep bright samples are usable as pigments. 
The places where the best qualities of terre verte are found are 



174 GREEN PIGMENTS. 

the Mendip Hills, as also many localities in France, Italy, and 
Cyprus. 

Terre verte is found in masses of a more or less compact 
character; some varieties are soft and easily powdered, others 
are harder and more vitreous in appearance. For use as a 
pigment, the mineral is ground up as fine as possible; sometimes 
it is levigated. 

Although varying somewhat, as might be expected in earthy 
pigments from various sources, yet there is a certain amount of 
resemblance between different samples of terre verte, as shown 
by the few analyses of this pigment available. Berthier gives 
the following analysis of terre verte, the source of which is not 
stated : 

Silica, Si0 2 , 51 '21 per cent. 

Ferrous oxide, Fe 0, . . . . 20 '72 ,, 

Soda, Na 2 0, 6'21 

Alumina, A1 2 3 , . . . . 7 '25 ,, 

Magnesia, MgO, .... 6*16 ,, 

Water, H 2 O, 4 '49 ,, 

Manganese, Mn 2 , . . . . trace. 

96-04 

This is not a very satisfactory analysis ; of what does the 4 per 
cent, of unaccounted for material consist 1 

A sample of terre verte from Cyprus, analysed by Klaproth, 
gave the following figures : 

Silica, Si 2 , ..... 51 '5 per cent. 

Ferrous oxide, Fe 0, . . . 20 '5 ,, 

Potash, K 2 0, 18'0 

Magnesia, MgO, . . . . 1'5 ,, 

Water, H 2 0, 8'0 

99'5 

A sample of terre verte from the neighbourhood of Rome was 
examined by the author, and found to have the following 
composition : 

Water, hygroscopic, H 2 0, . . 1 '450 per cent. 
Water, combined, H 2 0, . . . 3 '650 
Ferrous oxide, Fe 0, . . . 26 '870 



Alumina, A1 2 
Manganese, Mn 2 , 
Calcium oxide, Ca 0, 
Silica, SiO 2 , 



3-165 
trace. 

2-065 
52-120 



Magnesium oxide, Mg 0, . . . 10 '665 

99-985 



COBALT GREEN. 175 

This sample was very hard, and had a conchoidal fracture, a 
Avaxy lustre, and a soapy feel. Acids had but slight action on 
it. It was evidently a specimen of the mineral bronzite, which 
is essentially a ferrous magnesium silicate. 

Terre verte is of a pale bluish-grey tint, and has no great 
colouring power or body, being somewhat transparent; it mixes 
well with either oil or water, and is perfectly permanent, being 
unaffected by any length of exposure to light and air ; it is not 
altered by sulphur or sulphureous gases in any way. As a 
pigment terre verte has been used from very early times, being 
one of the best greens available to the early painters. Heat 
turns the colour of terre verte to a reddish-brown, the change 
being similar in nature to that which takes place when the 
ochres are heated. 

Sometimes greens having a copper base are offered as terre 
vertes: these are not permanent. 

COBALT GREEN. 

Cobalt green, Rinman's green, Zinc green is a compound of the 
oxides of zinc and cobalt, having an analogous composition to 
cobalt blue and being prepared in a similar manner. 

Preparation of Cobalt Green. Cobalt green can be 
prepared in several ways. 

(a) Sulphate of cobalt in solution is mixed with zinc oxide 
into a paste ; this is then dried and exposed to a red heat in a 
muffle furnace until the desired green tint has been developed, 
which will take from three to four hours. The tint of colour 
will depend upon the proportions of the cobalt salt and zinc 
oxide used ; 1 Ib. of cobalt sulphate to 5 Ibs. of zinc oxide will 
give a deep green ; with twice as much zinc oxide a grass green 
is obtained ; while if the proportions are 1 Ib. of cobalt sulphate 
to 20 Ibs. of zinc oxide a fine bluish-green is obtained, which 
forms a fair substitute for emerald-green. 

(6) Instead of the sulphate, the nitrate of cobalt may be used. 
1 Ib. of cobalt nitrate is mixed with 2 to 5 Ibs. of zinc oxide 
according to the depth of colour required ; the mixture is kept 
at a bright red heat in a muffle furnace for a few hours until the 
green has been fully developed ; it is then ground with water, 
and dried. The cobalt salt must be free from metallic impurities, 
such as iron, alumina, or tin. 

The principal difficulty in these two processes is that of 
ensuring a thorough mixture of the cobalt and zinc compounds ; 
if this is not properly done the green which is formed will not be 



176 GREEN PIGMENTS. 

of a uniform tint throughout the mass ; there will be dark and 
light places. The following processes avoid this difficulty by 
mixing solutions of the two metallic salts, thereby ensuring 
perfect admixture, then precipitating the oxides from the solution 
and finishing as in the above processes : 

J;) 1 Ib. of nitrate of cobalt or J Ib. of the chloride of cobalt 
6 Ibs. of sulphate of zinc are dissolved in 7 gallons of water ; 
a solution of carbonate of soda is then added as long as a 
precipitate falls ; the mixture filtered, and the precipitate of 
hydroxides of cobalt and zinc so obtained washed, dried, and 
heated as before. By varying the proportions between the zinc 
and cobalt salts the depth of colour of the resulting green can be 
varied to a great extent. 

(d) Instead of using the carbonate of soda to precipitate the 
solution of zinc and cobalt, there may be used either the phosphate 
or the arseniate of soda. Wagner states that the resulting green 
is purer, brighter, and less dense. The greens made in this 
way will contain phosphoric acid, which will give the greens a 
rather bluer tint. 

Cobalt green has a bright green colour of a slightly yellow 
hue. It is perfectly permanent when exposed to light and air, 
and is on that account a useful pigment. It can be mixed with 
all other pigments without being affected by them or altering 
them in any way. It is unacted upon by acids in the dilute 
state ; but strong acids decompose it, forming a blue solution. 
Alkalies have no action on it. 

Wagner gives the following analyses of cobalt green made by 
various processes : 

1. 2. 3. 

Per cent. Per cent. Per cent. 

Zinc oxide, Zn 0, . . 88-040 71 '93 71 '68 

Cobalt oxide, Co 0, . . 11-622 19-15 18'93 

Phosphoric oxide, P 2 5 , . ... 8 '22 8 -29 
Ferric oxide, Fe 2 3 , . . 0'298 

Soda, Na 2 0, 0'69 

100-000 99-99 98-90 

Owing to its cost cobalt green is not much used. Should it be 
possible to find a cheap source of cobalt this green might come 
into more general use, as its permanent qualities would give it 
superiority over many of the other greens. 

Besides the greens described above many others have been used 
on a small scale ; some are still used for special purposes, and 
others have been described by various chemists, but whether they 



CHINESE GREEN OR LOKAO. 177 

have ever been used on a practical scale is doubtful ; these will 
be briefly described. 

BRIGHTON GREEN is the name given to a pigment made 
by grinding together in the dry condition 7 Ibs. of copper 
sulphate, 3 Ibs. of acetate of lead, and 24 Ibs. of whiting ; during 
the grinding chemical decomposition sets in, resulting in the 
formation of a basic acetate of lead. It was a pigment of no 
great depth of colour or permanency. 

DOUGLAS GREEN. Mr. Thomas Douglas has described 
in the Chemical News> vol. xl., p. 59, a green prepared from 
barium chrornate. The latter compound, prepared in the usual 
way by mixing solutions of barium chloride and potassium 
chromate, is mixed with 20 per cent, of its weight of strong 
sulphuric acid, which partially decomposes it, forming a mixture 
of barium chromate, chromic acid, and barium sulphate; the 
mixture is dried and then calcined at a bright red heat in a 
crucible ; the chromic acid is thereby decomposed into the green 
oxide of chromium, which, being disseminated throughout the 
mass of barium sulphate and chromate, colours it green, forming 
a pigment possessing considerable body and permanency. 
Nothing definite is known as to its having been used as a 
pigment. 

CHINESE GREEN or LOKAO. This is a green pigment, 
as yet but little used, made from the juice of various Chinese 
species of buckthorn trees by extracting the juice from the berries 
by pressure, mixing this with alum, &c., and drying. It comes 
into commerce in the form of bluish-green slabs, which are easy 
to break, but somewhat difficult to powder. Chinese green con- 
tains from 27 to 47 per cent, of mineral matter, principally lime 
and alumina, and, probably, consists of the lake formed by the 
combination of those bases with the colouring principle of the 
juice from the buckthorn berries, named by Kayser lokaonic acid, 
C 42 H 48 O 27 . According to the same authority, the colouring 
principle consists of a glucose, which he calls lokaose, to which 
lie assigns the formula C 6 H 12 O 6 , and lokanic acid, a body having 
the composition C 33 H 36 O 21 . The colouring principle has also 
received the name lokain and the formula C 28 H 34 O ir . 

SAP GREEN. This pigment is prepared from buckthorn 
berries. Two methods are adopted in its preparation. In one 
the berries are allowed to ferment slightly by placing them in a 
warm place for a few days ; they are then pressed, the juice 
collected, and alum added in the proportion of from ^ an ounce to 
1 ounce per pound ; the mixture is then boiled down and 
evaporated to dryness at the boiling heat. Another plan is to 

12 



178 GREEN PIGMENTS. 

boil the berries in water for two or three hours with constant 
stirring ; the liquors are then strained through cloths in order 
to separate the woody and other insoluble particles ; the clear 
liquor is boiled down to a syrup, 5 ozs. of alum per gallon added 
to the syrup, and the mixture carefully evaporated to dryness. 
For some purposes the mass is left in the pulpy condition. 

Sap green is a dark yellowish-green pigment; when dry it 
breaks with a glossy fracture ; it is very transparent, and hence 
is not used as a body colour, but chiefly as a glazing colour ; 
another use for it is in colouring confectionery and beverages. 

It works well as a water-colour, but not as an oil-colour ; and 
fades on exposure to light. 

An analysis of sap green made by the author shows it to have 
the following composition : 

Water, 12'95 per cent. 

Mineral constituents, . . . . 10 '69 ,, 
Organic constituents, . . . . 76*36 

100-00 

Of the organic constituents a quantity equal to about 29-34 per 
cent, of the original colour are soluble in alcohol. Its com- 

Eosition and general properties somewhat resemble those of a 
ike. 

MANGANESE GREEN. This pigment was patented in 
1864 by Schad, who prepared it by the following process : 
14 parts of oxide of manganese, 80 parts of nitrate of barium, 
and 6 parts of sulphate of barium, are intimately mixed together. 
The mixture is heated in a crucible in a suitable furnace to a 
bright red heat until it has assumed a green colour ; it is then 
ground in a mill with water to a fine powder ; a small quantity 
of gum arabic, dextrine, or similar substance, amounting to about 
5 per cent, of the original material, is added, and the mass is dried 
at from 190 to 212 F. ; or it may be used in the form of a paste. 
Instead of the above mixture there may be used one of 24 parts 
of nitrate of manganese, 46 parts of nitrate of barium, and 
30 parts of sulphate of barium. The addition of the gum or 
dextrine is said to be essential for its stability, a factor which 
cannot but have an adverse influence on its value as a pigment, 
for which purpose it has probably not been used. It consists 
principally of manganate of barium. 

TITANIUM GREEN. This pigment is the ferrocyanide of 
titanium, prepared by mixing solutions of potassium ferrocyanide 
and of a titanium salt; the pigment must be dried at a low 
temperature, as decomposition sets in above 100 C. It has a pale 



ZINC GREEN. 179 

green colour, and was proposed as a substitute for the arsenical 
greens ; owing to its cost it has never come into use. 

ZINC GREEN. Under this name there is frequently sold 
greens made in a similar way to the Brunswick greens, by 
mixing together zinc chrome, Prussian blue, and bary tes ; such 
greens possess the advantage that they are not affected by sulphur 
as much as the Brunswick greens. They are best made in the 
dry way (see p. 151). 

The green lakes and the pigments made from coal-tar colouring- 
matters will be found described in the chapter on Lakes. 



180 



CHAPTER VI. 
BLUE PIGMENTS. 

Blue, as a colour, enters very largely into the decoration of 
objects, both alone and in combination with other colours, to form 
a large and very useful series of tints and shades. Although so 
important as a colour, yet there are few blue pigments ; but these 
possess the merit of being more permanent, and, therefore, more 
useful than any other group of pigments. The list of blue 
pigments includes ultramarine (a curious compound of silica, 
alumina, and soda, which was at one time obtained exclusively 
from natural sources, but is now mostly prepared artificially), 
and Prussian blue with its varieties (a most valuable blue, whose 
base is iron), which are the most predominant blue pigments 
used. Cobalt is the base of cobalt blue and smalts, while copper 
forms the basis of several unimportant pigments. 

ULTRAMARINE. 

Ultramarine is one of the most important pigments possessed 
by the painter ; being used in painting, in printing of all kinds 
(letterpress, wall-papers, calico), and in bleaching ; it is un- 
doubtedly the best blue for the laundry, and in soapmaking it is 
used to produce the blue mottled soap. 

Ultramarine has been known for centuries, but its extended 
use has only been possible during the last half century. Prior to 
about 1820 the natural supplies were small, and the processes so 
expensive that it could only be used by artists who did not find 
the cost prohibitive ; but about the year named, discoveries were 
made by several chemists, which resulted in ultramarine being 
made artificially at such a cheap rate that it is the cheapest blue 
pigment known j consequently, its consumption is now measured 
by tons. 

Natural Ultramarine. The source of natural ultramarine is 
a blue mineral, lapis lazuli, found in small quantities in Persia, 
China, Siberia, and a few other places. This mineral is found in 
streaks and small patches distributed through an earthy matrix 



ULTRAMARINE. 181 

or gangue, from which it has to be separated by mechanical 
means. The production of natural ultramarine has declined very 
much during the last fifty years, it having been displaced by 
artificial ultramarine ; but the mineral is still sought for in fair 
quantities, for use in the production of inlaid ornamental work, 
as the peculiar blue colour of the mineral cannot be obtained by 
other means. 

The process of extraction of the pigment from the mineral 
consists in grinding the mineral to a fine powder, after separating 
as much of the gangue as possible ; it is then mixed with a com- 
pound of resin, wax, and linseed oil, and the mixture put into 
cloths and kneaded under hot water ; the colour comes through 
the cloth into the water, several waters being used ; after the 
working, the waters are placed on one side for the colour to 
settle. The blue thus obtained varies in shade in the different 
waters ; that which settles out of the first water is the deepest in 
colour, and the brightest, and is sold as ultramarine ; that which 
comes from the last waters has a blue-grey colour, and is sold as 
ultramarine ash. After the colour has settled, it is usual to 
grind it still finer, so that the beauty of the pigment shall be 
developed as much as possible. No better process for extracting 
ultramarine has been devised, although it is so tedious and gives 
such poor results. Very little is now so produced, as the natural 
variety has been almost replaced in European and other 
countries by the artificial variety. 

The chemical composition and constitution of ultramarine early 
became the subject of research by chemists, which researches 
were partly undertaken with a view to its artificial production ; 
for it was recognised that, from the beauty of its colour and its 
permanent qualities, ultramarine would, if it could be produced 
cheap enough, have a wide field of use. Several analyses were 
made by different chemists, but these vary very much, owing, as 
is probable, to the difficulty of obtaining the pigment quite free 
from its matrix. Those by Clement and Desormes and by 
Gmelin, which are, perhaps, the most typical, are here given. 

Clement and 

Desormes. Gmelin. 

Silica, Si 2 , ... 35 '8 47 '306 



Alumina, A1 2 3 , 

Soda, Na 2 0, ... 

Sulphur, S, 

Calcium carbonate, Ca C 63, 

Calcium oxide, Ca 0, 

Sulphuric acid, H 2 S O 4 , . 

Water and loss, . 



34-8 22-000 

23'2 12-063 

3-1 0-188 



1 



1-546 

4-679 

12-218 



100-0 100-000 



182 BLUE PIGMENTS. 

It is evident that with such discrepancies in the analyses 
nothing could be satisfactorily inferred as to the chemical com- 
position and constitution of ultramarine, and it is no wonder 
that none of them led to its artificial production. 

Artificial Ultramarine. Early in the present century, soon 
after soda began to be produced on the large scale from salt by 
the Leblanc process, many persons noticed the formation of 
a substance resembling ultramarine in colour; Tessart and 
Kuhlmann recorded, in 1814, that they had seen this blue colour 
in a soda furnace. Vauquelin, on examining it, found it to be 
a compound of silica, alumina, lime, soda, and sulphur, and 
showed that it had a similar composition to ultramarine. It 
was recorded that it was formed only when sandstone was 
used in the construction of the furnace; when bricks were used 
it was not formed. 

Guimet, an eminent French manufacturing chemist, studied 
the production of ultramarine. In 1828 he succeeded in making 
it on a large scale, and obtained a prize of 6,000 francs, offered 
by the Societe d'Encouragement of France to any one who made 
ultramarine in a wholesale way. Guimet's process is still used 
by his successors, but has not been published. 

Gmelin also interested himself in the production of ultra- 
marine, and in 1828 he published an elaborate description of his 
method of making it. 

Kottig, the director of the Miessen Porcelain Works also, 
about the same time, observed the production of ultramarine in 
his furnaces, and, as the result of his researches, succeeded in 
making the pigment on a large scale ; the Miessen ultramarine 
was for many years one of the leading brands; the works are 
now closed. 

About 1834 Dr. Leverkus, working by Gmelin's process, 
started its manufacture in Germany at works which are still 
in existence. 

The great bulk of the ultramarine used is made in Germany ; 
there are two or three works in England, a few in France, and 
one in America. Several writers have given descriptions, more 
or less complete, of the process of making ultramarine ; but the 
best and most complete description is that by J. G. Gentele,* 
and more recently one by Kawlins.t 

Varieties of Ultramarine. There are two principal varieties 
of artificial ultramarine 1st, sulphate ultramarine, which is of 



Technologists, vol. xviii., pp. 389-411. 
Journ. Soc. Chem. Ind., 1887, p. 791. 



VARIETIES OF ULTRAMARINE. 183 

a pale greenish-blue colour ; 2nd, soda ultramarine, which has a 
violet-blue colour. Of the latter there are two varieties one 
contains more silica than the other, and is mostly used by 
paper-makers, owing to its resisting the action of acids and 
alum better, while the variety poor in silica is used for all other 
purposes. 

The materials used are nearly the same for both kinds, and 
comprise kaolin or china clay, sodium sulphate (Na 2 S O 4 ), 
sodium carbonate (Na 2 C 3 ), sulphur, coal or charcoal, rosin, 
quartz, and infusorial earth. All these are not used in the same 
operation ; some makers using one kind of mixture, others 
another. The quality of the materials is a matter of very great 
importance. 

The kaolin or china clay should be as free as possible from 
any earthy matrix ; a trace of lime has no injurious influence, 
but the clay must be free from iron, which has a tendency to 
dull the colour of the ultramarine. It has been found from 
experience that every sample of china clay does not give equally 

food results, although all may be pure and of good quality. 
t has been found that the relative proportions in which the 
silica, Si O 2 , are combined with the alumina, A1 2 O 3 , is a matter 
of some importance, and in china clays from different localities 
there is wide differences in this respect; then, again, a 
china clay which will work well for sulphate ultramarine 
will not do for soda ultramarine. For making sulphate 
ultramarine the china clay should contain the silica and 
alumina in the proportion of 2 silica to 1 alumina, 2 Si O 2 , 
A1 2 O 3 ; if the proportions much exceed these the shade will be 
poor, while if they reach those indicated by the formula, 3 Si O 2 , 
A1 2 O 3 , the clay will not make sulphate ultramarine. On the 
other hand, while almost all clays will make soda ultramarine, yet 
the best results are obtained with clays containing from 2J to 
3 parts of silica to 1 part of alumina ; the larger the proportion of 
silica, the redder the shade of the ultramarine made from it, and the 
more resisting power it has to the action of acids and alum. 
The china clay is prepared for use by a process of grinding and 
levigating, so as to obtain it in as fine a form and as free from 
impurity as possible. 

The sodium salts are used in the anhydrous state ; both should 
be as pure as possible, especially should they be free from iron, 
which has a most deleterious influence upon the shade of the ultra- 
marine made from the salts. Although other impurities are of 
small moment, still, where first-class ultramarine is required, it 
is best to purify the commercial products. 



184 BLUE PIGMENTS. 

The sulphur is the ordinary roll sulphur or brimstone. 

The coal and charcoal are the ordinary commercial varieties ; 
but the coal used must be free from pyrites. Both articles are 
ground before using. 

The quartz should be as free from impurities as possible ; the 
better the quality of the quartz, the better the quality of the 
ultramarine made from it. 

The infusorial earth or kieselguhr is the well-known com- 
mercial article. 

The rosin used is the best commercial variety obtainable. 

The manner in which these are mixed together depends upon 
the variety of ultramarine to be made, and it also varies in 
different works, each of which has its own formula, although 
there is not much variation in the essential points. 

Gentele lays down the following rules : 1st, That the soda 
used be sufficient to neutralise half the silica present in the 
kaolin or clay and silica used ; 2nd, that the proportions of soda 
and sulphur I e such as to produce a polysulphide of soda. 

ULTRAMARINE MANUFACTURE. There are two pro- 
cesses in use for the manufacture of ultramarine; the oldest, called 
the indirect process, is used for making both sulphide and soda 
ultramarines, and is the only process by which the former can be 
made. 

Indirect Process of Making Ultramarine. This consists 
of the two stages or operations, viz. : 

(a) The calcining operation. 

(b) The colouring operation. 

(a) Calcining Operation Manufacture of Ultramarine 
Green. A mixture of the ingredients named above is made ; 
if sulphate ultramarine is required, sulphate of soda is used ; if 
soda ultramarine is to be made, then soda carbonate is used. 
Some works use a mixture of the two soda salts. 

The various ingredients are ground together with water into 
a very fine paste ; the finer the grinding, the better will be the 
quality of the ultramarine ; after the grinding, the paste is 
dried. In some works the water is omitted, it being considered 
unnecessary, while the subsequent drying adds to the expense of 
making. 

The following are examples of the mixings used in different 
works : 

For sulphate ultramarine. 



ULTRAMARINE MANUFACTURE. 



185 



Kaolin, 
Sodium sulphate, 
Coal, . 


1. 
Parts. 
100 
83 
17 


2. 
Parts. 
100 
41 
17 
41 


3. 

Parts. 

100 
220 


Sulphur, 





13 


30 
35 



When sodium sulphate is used less sulphur is required ; in 
proportion as the latter is decreased so the proportion of the 
former must be increased. 

For soda ultramarine poor in silica 

Kaolin, ...... 

Sodium carbonate, .... 

Coal, 

Sulphur, 

For soda ultramarine rick in silica 



100 parts. 
100 

12 

60 , 



Kaolin, 

Quartz, 

Sodium carbonate, 

Sulphur, . 

Coal, 

Rosin, 



1. 

Parts. 
90 
10 

100 
60 
12 



1 

Parts. 
100 

90 

100 

4 

6 



The second recipe gives a dark ultramarine ; the more sulphur 
there is used in making soda ultramarines, the deeper is the 
shade of the blue produced; 
on the other hand, by re- 
ducing the quantity of sul- 
phur and silica the blue 
obtained is not so deep, 
but is rather more brilliant 
in hue. 

The mixture is then 
placed in crucibles, about 
6 inches by 4 inches in 
size, and fitted with lids, 
which are somewhat saucer- 
shaped, sothatthecrucibles 
can be piled one above an- 
other in a furnace. Fig. 
19 shows the shape of the Fig. 19. 

crucible and its lid. The 

mixture is packed rather tightly into these; sometimes what are 
called seggars are used, but the crucible form is better, as giving 




186 



BLUE PIGMENTS. 



a firmer pile when placed in the furnace. In some works open, 
flat, round capsules are used of such a size that they hold when 
j full, about 9 ozs. of the material, which forms a layer 1J to 1| 
inches thick; these are piled one above the other in a furnace 
capable of holding about 216 arranged in 9 layers of 24 capsules, 
each formed of lots of 6 by 4. 

The furnace in which these pots of material are placed varies 
in form in different works. Fig. 20 shows one form of ultra- 
marine furnace. The furnace chamber, B, is an almost exact 




Fig. 20. Ultramarine furnace. 

cube in form ; the back is completely closed in, and the front, 

C, is open, but is made up with firebricks when the furnace has 
been filled with the crucibles. The fireplace, A, is under the 
furnace chamber, the flames and heat in it pass through openings, 
e e e, in the floor of the chamber; similar openings, //, in the roof, 

D, of the chamber serve as outlets for the waste heat and gases 



ULTRAMARINE MANUFACTURE. 187 

of the furnace into the flue, E E. A number of these furnaces 
are built side by side and back to back, forming a range or bench 
of furnaces, but are not all worked together, for while some are 
being filled, others are being emptied, and others, again, are 
being heated. In some works a kind of muffle furnace, similar 
to Fig. 21, is used. 

After the furnace has been charged with the crucibles, the front 
is made up with bricks, and the interstices between these filled 
with a mixture of sand and clay, a small sight-hole being left so 
that the temperature of the furnace can be observed; if necessary, 
this sight-hole is stopped with an easily removable plug of clay. 
The temperature of the furnace is then slowly raised to a bright 
red heat, at which it is maintained for from 7 to 10 hours, the 
time varying with the nature of the composition and determin- 
able only by actual practice. Sulphate ultramarine requires a 
higher temperature than soda ultramarine ; if a muffle furnace 
is used, the temperature is often raised to a bright yellow for 
from 2 to 3 hours only. 

When the calcination is considered to be complete the fire is 
drawn and the furnace allowed to cool; this must be done as 
slowly as possible, and care must be taken that no air enters into 
the furnace during the cooling, because while hot the crude ultra- 
marine is very susceptible to the action of the oxygen of the air, 
and the yield as well as shade of the colour would be injured. 
When cold the crucibles are removed and the furnace is ready 
for another charge. This first burning of the ultramarine is a 
most important operation, and great care must be exercised in 
carrying it out ; access of air to the contents of the crucible must 
be carefully avoided; the temperature should not be too high 
nor too prolonged, as then the material would be overburnt and 
will not give a satisfactory blue; on the other hand, under- 
burning is just as bad, for then the colour will not be 
homogeneous. A furnace such as that shown can be charged 
three times per week. 

The colour of the burnt mass varies somewhat; usually it is of 
a green colour, mostly of a bluish tone (which is generally 
indicative of good burning), but sometimes it is of a yellowish- 
green shade, and at others it passes more into a blue, while, if 
not properly burnt, it will have a brown shade. 

The crude green ultramarine, which is somewhat cindery in 
appearance, is now thrown into water for the purpose of washing 
out all the soluble soda salts ; the last washings of one batch are 
often used as the first wash-waters of another batch for the 
purpose of economising the water. Whi}e&iil L^W&C^ 

f CFTHE ^' 

UNIVERSITT 



188 BLUE PIGMENTS. 

marine is ground up in mills into as fine a form as possible, in 
order to effect the completest practicable extraction of the 
soluble matter. The ground-up green ultramarine is then dried, 
when it is ready for the next operation. In this form it is sold 
under the name of green ultramarine for use as a pigment. 

The wash-waters contain a large proportion of sodium salts, 
chiefly in the form of .sodium sulphide. In many works it is 
customary to evaporate the liquors to dryness by means of the 
waste heat of the furnaces, and to use the dry residue for another 
mixing. 

(b) Colouring Operation Manufacture of Ultramarine 
Blue. The green ultramarine obtained in the first stage has 
now to be converted into the blue, which is done by heating it 
with sulphur in a furnace at a low temperature. 

There are three ways of carrying out this colouring operation. 
(1) On trays, (2) in a cylinder, and (3) in a muffle. 

1. Tray Method. A form of muffle furnace is built in which 
the muffle is filled with a number of trays or shelves. On these 
trays the green ultramarine is spread in layers of about an inch 
thick, and over them is sprinkled some sulphur; the muffle door 
is closed, the furnace is lighted, and the heat continued until the 
sulphur takes fire \ then the fires are drawn and the sulphur 
allowed to burn itself out, after which the crude pigment is taken 
from the muffle and finished in the manner described further on. 

2. Cylinder Method. It is also known as the German method. 
Small cast-iron cylindrical vessels are imbedded in brickwork 
over an ordinary fireplace ; these cylinders are closed at the back 
end, but open in front, which is fitted with a door made of 
wrought iron ; in this door are two apertures for the purpose of 
charging the cylinder with sulphur, while a pipe from the top of 
the cylinder carries off the gases produced by the burning of the 
sulphur. An agitator is fitted to the cylinder, by means of 
which its contents can be kept well mixed during the progress of 
the operation. 

From 27 to 34 Ibs. of the ground green ultramarine is charged 
into the furnace, the door closed and the fire lighted. When the 
temperature is sufficient to ignite sulphur, 1 Ib. of sulphur is 
thrown into the cylinder ; when this has burned away and fumes 
have ceased to issue from the cylinder, another pound of sulphur 
is thrown in and allowed to burn ; a small sample is now drawn 
from the furnace and its colour noted ; if not blue enough, more 
sulphur is thrown in at intervals until a sample taken out of the 
cylinder shows that the blue has properly formed ; after the 
cylinder has cooled down the pigment is scraped into a box, and 
is ready for the finishing operation. 



ULTRAMARINE MANUFACTURE. 



189 



During the whole of this operation the temperature of the fur- 
nace is kept at the proper heat, viz., that at which sulphur will 
burn, and the agitator is kept at work. 

3. Muffle Method. It is also known as the French method. 
In this method the green ultramarine is coloured by heating with 
sulphur in a muffle furnace. Such a furnace is shown in Fig. 21, 
which represents a longitudinal section of an ultramarine muffle 
furnace. The fireplace is shown at A, and is separated from the 




V^ 



Fig. 21. Ultramarine muffle furnace. 

muffle-chamber by a flued arch. The muffle, B, is made of 
earthenware, and is completely closed at one end, while the other 
is fitted with a door, D, so built into the furnace that none of the 
furnace gases can get into the interior of the muffle ; C is the flue 
of the furnace ; G is a kind of hood which collects all the vapours 
of the burning sulphur, and passes them into the flue, C, and so 
up the general chimney of the works. 

The green ultramarine is spread in a layer of about 1 to 
2 inches thick on the floor of the muffle, the door closed and the 
fire lighted ; when the temperature is high enough for sulphur to 
burn, a shovelful of that substance is thrown into the muffle and 
stirred with an iron rod ; when the first shovelful has burnt out, 
more sulphur is added from time to time, until a sample of the 
colour taken out of the furnace shows that it has acquired the 



190 BLUE PIGMENTS. 

desired blue colour. The muffle is more rapid in working than 
the cylinder. The blue is raked out of the furnace and is 
finished in the usual manner. 

The indirect process, while yielding a very good quality of 
ultramarine, labours under the disadvantages of making it in 
small quantities only at a time, and of being attended with a 
large loss of material in the operations. 

The Direct Process of Ultramarine-making. The disad- 
vantages of the indirect method induced the manufacturers to 
seek a new method, by which larger quantities could be made at 
one time, and in which the loss of material would not be so 
great ; the labours of the chemists who have been engaged on 
this object were rewarded with success by the discovery of a 
direct method having the desired advantages. The direct method 
can, however, only be used for making the soda ultramarine, but 
as this happens to be the principal variety, the one disadvantage 
attending the process is practically of no moment. 

The direct process can be carried out either in (1) muffle 
furnaces, or (2) in crucibles. 

1. Muffle Method. A mixture of kaolin, sodium carbonate, 
sodium sulphate, sulphur, sand, rosin, or charcoal is made, the 
proportions varying in different works, but approximating to 
those already given. 

The mixture, reduced to the finest state so as to ensure the 
most intimate union of the ingredients, is placed in layers of from 
2 to 3 inches thick, and firmly pressed down on the floor of the 
muffle ; a charge weighs about 45 Ibs. The surface of the charge 
is covered with fireclay tiles, and the spaces between these luted 
with mortar ; at the front of the muffle one of the tiles is left 
loose, so that, when required, it can be raised to admit of samples 
being withdrawn for testing. The front opening of the muffle is 
now made up, a small aperture being left for the purpose of 
observing the temperature of the muffle and for drawing out 
samples from time to time. 

The furnace is now heated, at first slowly, towards the last 
more strongly, so that in about 8 or 9 hours it has attained a 
dull red heat, at which temperature it is maintained for 2-4 hours, 
and then raised to a bright red heat until the end of the operation. 
A sample is now withdrawn from the furnace through the 
hole in the door and a corresponding hole in the tiles ; this 
sample is placed between two tiles as quickly as possible, and a 
second sample is taken out and placed on the top of the tiles ; 
when the samples have cooled, the colour of the samples are 
compared. If the operation is finished, the colour of the second 



ULTRAMARINE MANUFACTURE. 191 

exposed sample will be of the blue colour, while that of the first 
sample (the covered one) will be of a blue-green colour ; when 
this is found to be the case, the fires are drawn and the furnace 
and its contents allowed to cool down, care being taken that no 
air enters into the furnace ; or, to make quite sure, the heat is 
usually maintained for another hour. Should the trial samples 
have a brown colour, the mass has been insufficiently heated, 
and the temperature of the furnace is raised a little higher. 

When the furnace is opened the ultramarine is found to be in 
two layers an upper one of a bright blue colour, and a lower 
one of a bluish-green ; these are separated and finished in the 
usual way, the upper layer forming the best and the lower layer 
an inferior quality of ultramarine. 

Although the quality of the blue produced by this method is 
good, yet the quantity capable of being produced is small ; 
therefore it is not much used, and the crucible method described 
below has replaced it to a large extent. 

2. Crucible Method. The method most largely employed for 
the production of ultramarine is that known as the crucible 
method, and is carried out as follows : 

A mixture is made of 

Kaolin, 100 parts. 

Sodium carbonate, 90 

Sulphur, 110 ,, 

Charcoal, 20 

A variable quantity of infusorial earth (from 20 to 30 parts) are 
added, according as the ultramarine has to be rich or poor in 
silica ; in some works 6 parts of rosin are added. All these 
ingredients are ground together into a homogeneous mass j this 
is a point of great importance. The mixture is loosely packed 
into crucibles fitted with flat lids, which are luted on by means 
of mortar. When the mortar luting is dry the crucibles are 
piled in ovens large enough to hold from 400 to 500 crucibles, 
and, in shape, not unlike that described for the first stage of the 
indirect process. After all doors and openings into the oven arc 
made up it is fired to a bright red heat for several hours, the 
length of time varying considerably and depending upon a 
number of factors, such as the state of the weather, the com- 
position of the mixture, &c. Experience is the only school in 
which an ultramarine-maker can learn how to regulate the time 
required. 

After the heating, all apertures are carefully closed, so as to 
exclude air, and the furnace allowed to cool for four or five days; 
the oven is then opened, the crucibles withdrawn and opened, 



192 BLUE PIGMENTS. 

the contents turned out, and the badly-burnt pieces carefully- 
separated ; the good portions are ready to be finished. 

The changes which go on during the heating of the mixture 
are both curious and interesting. The mixture when first put 
into the crucibles is of a greyish colour, but during the process 
of burning it passes through quite a series of colour-changes 
brown, green, blue, violet, red, and white. The brown appears 
with the blue flames, due to the burning of the sulphur ; it is a 
fine chocolate brown, but is very unstable ; on exposure to the 
air it enters into combustion. Many efforts have been made to 
preserve it, but these have been fruitless. The green, which 
is the next change, begins to form when the sulphur has 
ceased to burn ; like the brown it is unstable, as the substance 
burns on exposure to the air. Following the green comes the 
blue, which is formed when the temperature has reached about 
700 C., or a bright red heat ; when the temperature gets higher 
the colour changes to a violet. With still higher temperatures, 
first a red, then a white variety is formed. These changes are 
due to oxidation ; when the white ultramarine is heated with 
reducing agents, such as carbon, the colours are re-formed in the 
reverse order to that in which they first appeared. 

The form of furnace to be used in burning the ultramarine is 
not a matter of importance, the operation can be effected in a 
reverberatory furnace, in a muffle furnace, in earthenware pots, 
in ovens, or in any convenient apparatus. 

Finishing Ultramarine. By whatever process the pigment is 
prepared it comes from the furnaces in the form of a gritty, 
somewhat cindery-looking, blue mass containing a large quantity 
of soluble sodium salts, and in this condition is unserviceable for 
use as a pigment. To fit it for this purpose the crude ultra- 
marine has to undergo a finishing process, which has for its 
object to purify the colour and to develop the hidden beauty of 
the pigment. The process of finishing is essentially one of 
washing and levigation. The crude ultramarine is thrown into 
grinding mills where it is ground with water, this grinding being 
done as thoroughly as possible, as on it depends to a very large 
extent the excellence of the pigment as regards colouring power 
and fineness. After the grinding, the wet ultramarine is run 
into large tubs, where it is treated with hot water, or even boiled 
with water, so as to make sure that all the soluble contents of the 
crude ultramarine are dissolved out. The ultramarine is now 
allowed to settle and the liquor run off; this contains sodium 
sulphate which may be recovered by evaporation and used in 
making new batches of ultramarine. Then clean water is again 



ULTRAMARINE MANUFACTURE. 193 

run on to the pigment, which, after being thoroughly stirred up, 
is again allowed to settle and the water again poured off; this 
washing is repeated several times. 

The wet ultramarine is now ground in grinding mills specially 
constructed for grinding wet materials very finely; such mills will 
be found described in another Chapter. This grinding is import- 
ant and takes several hours; the length of time depends upon the 
use to which the ultramarine is to be put. The finer qualities, 
which are used in calico-printing and letterpress and lithographic 
printing, and must be very fine, require the longer grinding; they 
are sold under the name of calico-printers' ultramarine ; painters 
do not require so fine a quality, and for this the wet ultramarine 
is not subjected to lengthy grinding. Another method of 
separating the different qualities of ultramarine is by levigation, 
which forms an essential part of the process. 

The wet ultramarine as it comes from the grinding mills is run 
into large tubs of water, in which it is thoroughly stirred and 
then allowed to settle for two hours ; this allows the coarser 
particles to subside, while the finer particles still remain in 
suspension, and are run into other tubs, where they are allowed 
to settle. The coarse particles in the first tub are run into the 
mills again to be re-ground with another batch of crude ultra- 
marine. The particles which settle in the second tubs are 
collected, dried at a gentle heat, and sent into the market. In the 
water of the second tub there still remains some fine ultramarine ; 
this is run into a third tub, where it is allowed to settle, and, 
after drying, is sold as a fine quality. Frequently, there still 
remains in the last waters some very fine ultramarine, even when 
the tubs have been allowed to stand for a month to settle ; 
by adding a little lime water, which causes an aggregation of the 
particles, this can be collected by filtering. 

Before being sold the dry ultramarine is, in many works, 
subjected to a process of sieving, which separates the coarser 
particles and yields the pigment in the form of an impalpable 
powder; the finer qualities should have a buttery feel when 
rubbed between the fingers. 

The shade of the finished ultramarine depends upon several 
factors, such as the proportions of the constituents used in the 
mixings, the perfection of the burning operations, and the 
fineness to which the pigment has been ground ; as it is impos- 
sible to regulate each of these factors with mathematical accuracy, 
it follows that the shade of the finished colour must vary from 
time to time ; and as this variation is objectionable the makers 
overcome it by having a number of standard or type colours or 

13 



194 BLUE PIGMENTS. 

shades, to which standard they bring up all batches by a process 
of blending and mixing different shades together so as to obtain 
the marketable brands. 

Wet Methods of Making Ultramarine. Many attempts have 
been made to prepare ultramarine by wet processes, but mostly 
without any success. Knapp's process, given in the Jour, fur 
Praktisch. Chem., 1885, p. 375, consists in first roasting a mixture 
of kaolin, sodium carbonate, and sulphur to such a temperature 
that the roasted mass has a brown colour, at which point it is 
maintained until the kaolin is completely decomposed ; after 
cooling, the mass is digested in a solution of sodium persulphide. 
The defect of this process consists in the small margin there is 
between success and failure j if the colour of the roasted mass be 
allowed to pass beyond the brown, the colour of the finished 
ultramarine begins to deteriorate ; and if it becomes red, then no 
blue is produced when the mass is digested with the persulphide 
under these conditions the process can hardly become a com- 
mercial success. 

The colour of the finished product is not quite equal to that of 
ultramarine made by the dry methods ; it is, however, not much 
inferior. 

PROPERTIES OF ULTRAMARINE. Ultramarine is one 
of the most important pigments at the command of the painter. 
As a pigment it is perfectly permanent when exposed under all 
ordinary conditions, being perfectly fast to light and air, the only 
destructive agents being acid vapours which rapidly decolorise 
it. It can be mixed with all the ordinary vehicles used by 
painters and with most other pigments without being changed 
thereby or itself causing any change. The only exceptions are 
those pigments containing lead or copper, which, owing to their 
forming black sulphides with sulphur, are liable to become dis- 
coloured when mixed with ultramarine ; the rate of change of 
such mixtures as ultramarine with chrome-yellow or emerald- 
green is very variable ; sometimes the mixture will change colour 
very soon, at other times the mixture will keep its colour for a 
considerable time ; much depends upon the quality of the pig- 
ments and the care with which they have been made. 

Ultramarine is distinguished by its pale but pure tone and by 
its tint of blue being quite different from that of all other blue 
pigments. The soda ultramarines are of a violet-blue shade, the 
variety rich in silica having the darkest and deepest tint ; the 
sulphate ultramarine is of a pale greenish-blue tint and is the 
palest blue pigment made, resembling blue verditer in tint. 

The most characteristic property of ultramarines is their being 



COMPOSITION OF ULTRAMARINE. 



195 



readily acted upon by acids ; the colour is discharged and the 
pigment decomposed, sulphuretted hydrogen being evolved and 
sulphur deposited. All acids have this property, even weak 
organic acids, such as acetic acid, tartaric acid, <fcc., this distin- 
guishes ultramarine from all other blue pigments; on the other 
hand, it prevents the use of ultramarine wherever there is the 
least chance of its coming into contact with acid influences, 
which are, sooner or later, sure to destroy the colour. Of the 
varieties of ultramarine, the sulphate is the most readily decom- 
posed, while the highly silicated soda variety is the most stable 
of the soda ultramarines. Boiled in strong nitric acid, there is, 
first, a decoloration, and then a deposition of sulphur; afterwards 
the sulphur is dissolved and a residue of gelatinous silica is left 
behind. Alkalies have no action on ultramarine. When boiled 
in alum, ultramarines take a more violet tone ; the sulphate 
variety is the most readily changed, while the highly silicated 
soda ultramarine resists the action most; the latter variety is 
therefore used by papermakers, because, owing to their having to 
use alum or sulphate of alumina in sizing their papers, they 
require an ultramarine which will not change much, if anything, 
under the influence of those bodies. Heat has no action on 
ultramarine. 

COMPOSITION OP ULTRAMARINE. Ultramarines 
are compounds of silica, Si O 2 , alumina, A1 2 O 3 , soda, Na 2 O, 
sulphur, S, and sulphur oxide, S O 3 . The last, although present 
in almost every sample of ultramarine, is not an essential con- 
stituent of the colour. 

The following are some analyses of ultramarines, mostly by 
the author, which will show the average composition of these im- 
portant pigments : 

ANALYSES OF ULTRAMARINES. 





Sulphate. 


Soap- 
makers' 


Calico- 
Printers'. 


Paper- 
makers'. 


Qreeu. 


Silica, Si 2 , 


49-685 


40-647 


40-885 


45-420 


38-52 


Alumina, A1 2 Os, . . . 
Sulphur, S, 


23-000 
9-234 


25-047 
12-953 


24-110 
13-740 


21-147 
11-624 


28-94 
8*30 


Sulphur trioxide, S 3 , . . 
Soda, Na 2 O, 


2-464 
12-492 


4-814 
14-264 


3047 
15-618 


5-578 
9-906 


23*68 


Water, Ho 0, 


3-125 


2-275 


2-600 


6-325 


















100-000 


100-000 


100-000 


100-000 


99-44 



196 BLUE PIGMENTS. 

The soap-makers', calico-printers', and paper-makers' ultra- 
marines are of English make, the others of Continental make. 
The analyses of paper-makers' and soap-makers' ultramarines 
show the difference between the two varieties of soda ultra- 
marines ; the first named is rich in silica, while the other is poor 
in silica ; the soap-makers' and calico-printers' samples are 
evidently identical in composition, but the latter is much finer 
than the former. The analysis of green ultramarine shows the 
difference between the green and blue ultramarines. 

CONSTITUTION OP ULTRAMARINE. One of the pro- 
blems chemists have endeavoured to solve has been how the 
various constituents of ultramarine are combined together, but it 
is still unsolved, and will probably remain so for some time to- 
come; the difficulty of solving it seems to be the inability to 
effect the substitution of particular groups of elements in it in 
the same manner as can be done in organic chemistry, where, 
even in complex molecules, the power of replacing one group by 
another enables one to form some conception as to the actual 
constitution of the compound. It is true that the sodium in 
ultramarine can be replaced by silver and other metals so as to 
form varieties of ultramarine, and that the sulphur can be re- 
placed by selenium or tellurium; but these replacements throw no 
light on the problem, for they are simply replacements of one 
element by another, not of groups of elements. 

Many chemists, e.g., Wilkins, Hofmann, linger, Endeman, and 
Elmer, have worked on this question. 

Hofmann's theory of the constitution of ultramarine is, perhaps, 
nearest the truth. Hofmann was head of the Marienberg Ultra- 
marine Works and did much to throw light on this subject ; he 
considered ultramarine to be a double silicate of alumina and soda 
combined with bisulphide of sodium. The formula assigned 
to the soda ultramarine poor in silica was 4 (A1 2 Na 2 Si 2 O 8 ) 
+ Na S 4 , and to that rich in silica, 2 (A1 2 Na 2 Si 3 10 ) + Na 2 S 4 . 

Endeman considers that the ultramarines contain a colour- 
nucleus (an oxysulphide of alumina and soda) disseminated 
through a double silicate of alumina and soda. 

The colour-nucleus of white ultramarine (which may be re- 
garded as the parent body) has the formula Al Na 4 O 2 S 2 ; the 
action of sulphur upon this is to remove soda and to form green 
ultramarine, which contains the nucleus, Al 2 Na 2 S 2 O 3 ; this, by 
oxidation, can be converted into A1 2 Na 4 S 3 O 3 which has a jet 
green colour ; by burning with sulphur, this is converted into the 
nucleus of the blue variety, which has the formula A1 2 Na 2 S 3 O 3 . 
The base through which the colour-nucleus is distributed is of 



CONSTITUTION OF ULTRAMARINE. 197 

variable composition ; Endeman gives the formula of one variety 
as 16 Si O 2 , 3 A1 2 O 3 , 5 Na 2 O. But all this is open to great doubt. 

The most competent authorities consider that the green ultra- 
marine is not a true chemical compound, but a combination of 
the blue with sodium salts; because, by simply boiling with water, 
it is converted into the blue ultramarine, while soluble sodium 
salts are found in the water ; on the other hand, by heating the 
blue ultramarine with sodium sulphate and charcoal, it is con- 
verted into the green ultramarine. 

Gueckelberger, one of the most recent writers on the subject, 
confirms the figures given by Hofmann ; but considers that the 
ultramarines are derived from a typical compound containing 
Si 18 ; thus, for the variety rich in silica he proposes the formula 
Si lg A1 12 Na 20 S 6 O 62 ; while the variety poor in silica has the 
formula Si 18 A1 18 Na 20 S 6 O n . It is doubtful whether ultramarines 
have the complex composition here assigned to them and, more- 
over, no light is thrown on their constitution. 

ASSAY AND ANALYSIS. As there is so much differ- 
ence between various makes of ultramarines, it is necessary to 
assay for colour, fineness, body, <fec. Those ultramarines which 
are to be used by paper-makers should be tested for their power 
of resisting the action of alum ; this can be done by taking about 
5 grammes and boiling in a solution of alum of about 5 per cent, 
strength. To see what change of colour may have taken place, 
5 grammes of the colour should be shaken up with clean water 
and the two wet samples compared together ; any change brought 
about by the alum can then be readily detected. 

It is rarely that a complete analysis of ultramarine is required; 
in such an event, the following scheme can be adopted : 

For Water. Heat 2 grammes in a weighed crucible for about 
half an hour over the Bunsen flame; the loss in weight is the 
amount of water present. 

For Silica, Si O 2 . Treat 2 grammes with hydrochloric acid 
until the colour is completely destroyed ; evaporate the mixture 
to dry ness and gently ignite the residue ; treat the dry mass with 
hydrochloric acid, filter off the insoluble silica, well wash it, then 
dry, and burn in a weighed crucible; the increase in weight minus 
the weight of the filter-paper ash is the weight of the silica. 

For Alumina, A1 2 O 3 . To the filtrate from the silica add am- 
monia in slight excess, boil gently, then filter, and treat the 
precipitate of alumina as the silica, 

For Soda, Na 2 O. To the ammoniacal filtrate from the alumina 
add sufficient sulphuric acid to neutralise the ammonia, then 
evaporate to dryness in a weighed basin and ignite the 



iSITT 



198 BLUE PIGMENTS. 

until all ammoniacal fumes have been given off; weigh the residue 
of sodium sulphate, and multiply this weight by 04366 to ascer- 
tain the weight of the soda, Na 2 O. 

Total Sulphur. Treat 2 grammes of the ultramarine with a 
mixture of 2 parts of nitric acid and 1 part of hydrochloric acid, 
until the colour is completely decomposed and only a transparent 
mass of silicate is left ; filter this off, and to the nitrate add a 
solution of barium chloride in excess, boil and filter, wash the 
precipitate well, dry, burn, and weigh it in a crucible. To find 
the weight of sulphur, multiply the weight of barium sulphate so 
found by 0-13734 ; from this deduct the weight of sulphur present 
as sulphuric acid to find the quantity of sulphur present as 
sulphide. 

For Sulphur as Sulphuric Acid. Weigh out 2 grammes of 
ultramarine, treat with dilute hydrochloric acid, filter off the 
precipitated sulphur and silica, and precipitate the filtrate with 
barium chloride, treat the precipitate as in the last. To find the 
amount of sulphur trioxide present, multiply the weight of the 
barium sulphate so found by O34335. 

ULTRAMARINE DERIVATIVES. It has been stated 
above that some of the constituents of the blue and green ultra- 
marines can be substituted by other analogous bodies, such as 
selenium for the sulphur or the sodium by silver ; in this way 
other ultramarines can be prepared, but, as a rule, they are only 
of scientific interest, as their colour is of no technical moment, as 
will be seen later on ; hence they are not made on a large scale ; 
still their production may ultimately throw light upon the 
question of the chemical constitution of ultramarine, and the 
fact of their formation must be faced by all chemists who essay 
to deal with this question. There are one or two coloured 
derivatives of ultramarine which are used to a limited extent ; 
these are the violet and red ultramarines. 

VIOLET ULTRAMARINE. This product can be made 
from either the green or blue ultramarines, from which it differs 
by containing less sulphur and more alumina. It can be made 
in several ways. Zeltner makes violet ultramarine by submitting 
either the blue or green varieties to the temperature of about 
300 0., and passing chlorine gas over them ; at first the colour 
is, if the blue is used, turned green, then this becomes dark red; 
at this point the operation is stopped and the red product is 
boiled in an alkaline solution until it turns violet ; after which 
it is washed. Instead of using dry chlorine at a temperature of 
300 C., the green or blue varieties may be heated in a mixed 
current of steam and chlorine at a temperature of 160 to 180 C., 



VIOLET AND RED ULTRAMARINES. 199 

until the colour is developed ; after which it is washed with 
water to free it from the sodium chloride formed, and dried. 

In another method, devised by Hofmann, blue ultramarine 
mixed with about 2 -5 per cent, of ammonium chloride; the 
mixture heated to a temperature of 200 0., exposed to the air 
until the violet colour is properly developed, and the mass allowed 
to cool slowly ; when cold, it is washed thoroughly and dried. 

Yiolet ultramarine has very similar properties to the blue 
variety, and is similarly decomposed by acids ; boiling in alkalies 
changes the colour to blue. The shade of the pigment is a very 
pale reddish-violet. This pigment is not used to any great 
extent owing to its want of colouring power. In composition it 
resembles the blue varieties ; a sample analysed by the author 
contained : 

Silica, Si 2 42 '010 per cent. 

Alumina, A 1 2 3 , .... 26 '300 ,, 

Sulphur, S, 9-235 

Sulphur trioxide, S O s , . . . 3 '140 ,, 

Soda, Na 2 0, 17 '905 

Water, H 2 0, 1-350 

100-000 

RED ULTRAMARINE. Zeltner prepares the red ultra- 
marine by exposing the blue variety at a temperature of 130 to 
150 C. to the action of the vapours of nitric acid, when he 
obtains deep or dark red, or light rose or pink shades, according 
as the acid vapours are dilute or strong. Hofmann prepares the 
red ultramarine by passing dry hydrochloric acid gas over either 
the blue or violet varieties until the proper colour is developed, 
when the mass is washed and dried. 

OTHER ULTRAMARINES. By using boracic acid instead 
of silica a boron ultramarine of a blue colour is obtained. Yellow 
ultramarine is made by heating the blue ultramarine with a 
solution of silver nitrate, in sealed tubes, at a temperature of 
120 C. for 15 hours; the sodium is replaced by silver, and the 
new pigment contains 46*5 per cent, of silver. 

The blue ultramarine heated with silver chloride turns green, 
taking up silver in the process. The yellow silver ultramarine 
heated with sodium chloride loses some of its silver, turning 
green ; if the sodium chloride is replaced by potassium chloride a 
bluish-green potassium ultramarine is formed. If barium chloride 
is used a yellowish-brown barium ultramarine is obtained ; in the 
same way zinc chloride yields a violet zinc ultramarine, and 



200 BLUE PIGMENTS. 

magnesium chloride a grey ultramarine. None of these products 
have any technical value. 

The sulphur can be replaced by selenium, when brown and 
purple ultramarines are obtained. 

'PRUSSIAN BLUE. 

Prussian blue, or Berlin blue, or Chinese blue, is, next to 
ultramarine, the most valuable blue pigment in use for painting 
and other purposes for which pigments are used. It was dis- 
covered in the early part of the last century (about 1704) by a 
Berlin colour-maker, named Diesbach, by accident, as many such 
discoveries have been made. Diesbach was making Florentine 
lake, and for this purpose he used a solution of cochineal, which 
he mixed with alum and copperas (ferrous sulphate) and pre- 
cipitated with an alkali ; in the particular instance which led to 
the discovery of Prussian blue he used an alkaline solution 
(which had been used to purify some Dippel's oil made by dis- 
tilling ox blood), and instead of getting a red lake he got a blue. 
Diesbach followed up this discovery and found that the blue 
could be got by calcining blood with alkali, and, after lixiviating 
the mass, precipitating the liquor with a solution of copperas. 

The technical manufacture of this pigment was further de- 
veloped by a London colour-maker, named Wilkinson, from 
whom the colour was named "Wilkinson's blue," a name which 
is now obsolete. Wilkinson prepared the colour by first de- 
flagrating a mixture of tartar and saltpetre, and calcining the 
residue with dried blood ; the fused mass was lixiviated with 
water, and to the lye so obtained a solution of alum and copperas 
was %lded ; the resulting pale blue precipitate was treated^jvith 
hydrochloric acid to develop the blue. 

Since the time of Diesbach and Wilkinson the composition of 
Prussian blue has been the subject of numerous researches by 
chemists, so that very little remains to be learnt as to the com- 
position and constitution of this blue. 

Prussian blue is a compound of iron, carbon, and nitrogen ; 
the carbon and nitrogen are combined together in the form of the 
radicle cyanogen, C N, which is the characteristic element of a 
group of compounds, of which Prussian blue is a member, known 
as cyanides. The iron exists in the blue in two forms ; one in 
combination with the cyanogen in an acid condition, the other in 
the basic condition. When Prussian blue is boiled with a 
solution of potash it yields oxide of iron (which remains as an 
insoluble red mass), and a yellow solution, which, on being 



PRUSSIAN BLUE. 201 

allowed to crystallise, deposits yellow tabular crystals of a com- 
pound of potassium, iron, carbon, and nitrogen, originally known 
as yellow prussiate of potash ; the iron in this exists in com- 
bination with the carbon and nitrogen in an acid sCate, forming 
the radicle known as ferrocyanogen, Fe C 6 N 6 . The chemical 
name of the yellow prussiate of potash is potassium ferrocyanide, 
K 4 Fe C 6 N 6 . Besides the yellow prussiate there is another, the 
red prussiate or potassium ferricyanide, K 3 Fe C 6 N 6 . These two 
compounds differ from one another in their colour and in the 
reactions which they give with iron salts. With ferrous salts the 
ferrocyanide gives a bluish- white precipitate of ferrous ferro- 
cyanide ; while with ferric salts a deep blue (Prussian blue) pre- 
cipitate of ferric ferrocyanide is obtained. With ferrous salts 
the ferricyanide gives a deep blue precipitate of ferrous ferri- 
cyanide (Turnbull's blue); with ferric salts no precipitate is 
obtained, but the colour of the solution becomes a little darker. 
The production of Prussian blue is a most characteristic reaction 
of iron, no other metal is capable of producing it, and very 
minute traces of iron in a solution can be detected by adding a 
few drops of a solution of potassium ferrocyanide. 

It has been ascertained by various observers that the precipi- 
tates obtained by adding solutions of the prussiates to solutions 
of iron salts contain potassium as an essential part of their 
composition, and it is difficult, although possible, to rid them 
of this potassium. Thus the bluish-white precipitate is really 
potassium ferrous ferrocyanide, K 2 Fe C 6 N 6 ; Prussian blue is 
potassium ferric ferrocyanide, K 2 Fe 2 2{Fe C 6 N 6 ; Turnbull's blue 
is potassium ferro-ferricyanide, K Fe Fe C 6 N 6 . 

Prussian blue and Turnbull's blue have exactly the .same 
comjfrosition, but their constitution is different ; the one being 
a ferrocyanide and the other a ferricyanide. Skraup,* Reindel,t 
KekulejJ and other authorities consider that they are identical; 
but that they are different is proved by the fact that when the 
alkali is eliminated from them the residual blues are of different 
composition, Prussian blue having the composition Fe 7 C 18 N 18 , 
and Turnbull's blue the composition Fe 3 C 12 N 12 ; then the 
shade of the two blues is different, Prussian blue is a greenish- 
blue, while Turnbiill's blue is a violet-blue. 

COMMERCIAL PRUSSIAN BLUE. In commerce 
several varieties of Prussian blue are sold under the names of 

* Skraup, Liebiffs Annalen, clxxi., p. 371. 
tReindel, Jour. PraL Chemie, cii., p. 38. 
+ Kekule, Lehrbuch. Organ. Chem. 

Williamson, Mem. Soc. Chem., iii., p. 125; Reynolds, Jour. Chem. Soc., 
li., p. 644. 



202 BLUE PIGMENTS. 

1, Chinese blue ; 2, Prussian blue ; 3, soluble blue ; 4, Antwerp 
blue ; 5, Brunswick blue, and others of less importance. There 
are several synonyms, such as Berlin blue and Paris blue. 
Turnbull's blue is a name rarely met with now. No special 
distinction is made between a blue obtained from the red prus- 
siate or a blue obtained from the yellow prussiate of potash. 

CHINESE BLUE. This is the name given to the best quali- 
ties of Prussian blue, and in the manufacture of which every care 
is taken to obtain a product of good colour. Chinese blue is 
especially characterised by being in pieces or powder having a 
fine bronze lustre, the pieces break with a peculiar conch oidal 
fracture, and the fractured surfaces show the lustre or bloom ; 
it is completely soluble in oxalic acid. Chinese blue is largely 
used by calico-printers and dyers. It is a blue of a greenish 
shade. 

Chinese blue is made as follows : 1 cwt. of ferrous sulphate 
(green copperas), as free from insoluble oxide as possible, is 
dissolved in cold water, and to it is added 10 Ibs. of sulphuric 
acid. This solution must be made as required, as it soon 
begins to oxidise, and to deposit oxide of iron, while the 
liquor will then not make good Chinese blue. One cwt. of 
yellow prussiate of potash is dissolved in water. 

The solutions should be made as dilute as possible, not less 
than 30 to 35 gallons of water for each cwt. of material ; even 
weaker solutions are preferable, as these yield finer precipitates 
than strong solutions, and so facilitate the production of the 
lustre on the finished blue. 

On mixing the solutions a bluish-white precipitate is obtained, 
which is allowed to settle, and the clear liquor poured off; to 
the residual blue is added first, a thin cream of 20 Ibs. of 
bleaching powder with water, which is thoroughly mixed with 
the precipitate, and then some hydrochloric acid, and the blue 
colour gradually develops. It is allowed to settle, the top liquor 
run off, and the blue well washed with water, and drained on a 
filter; the wet mass is then pressed into drying-pans, and slowly 
dried in the dark, at a temperature not exceeding 120 to 130 F. 

It is important that the oxidation of the precipitate first 
obtained be done by purely chemical means, and not by the 
agency of the oxygen of the air ; in the former case a pure blue 
colour is obtained, while in the latter case oxide of iron is 
mixed with the blue, and materially influences the tint of the 
pigment. The best oxidiser and the cheapest is bleaching 
powder ; nitric acid may be used, but it is more costly and not 
more efficient than bleaching powder. It will be found best to 



PRUSSIAN BLUE. 203 

add the oxidiser in small quantities, as when used all at once 
there is generally an escape of chlorine, owing to this body being 
evolved rather more rapidly than the blue can take it up, which 
not only increases the cost of production, but also deleteriously 
affects the workmen's lungs. No part of the blue should be 
allowed to come in contact with the atmosphere before it is iully 
oxidised. The slower the colour is dried, the better and finer is 
the lustre of the finished product. 

Instead of adding the bleaching powder after precipitating, it 
may be added to the iron solution to oxidise that to the ferric 
condition ; the blue obtained is not of so green a tint, being, if 
anything, a little more violet. 

If a blue with a violet tint is required it may be made by 
dissolving 1 cwt. of copperas in water and adding first 10 Ibs. of 
sulphuric acid, and then a solution of 1 cwt. of the red prussiate 
of potash. The precipitate is collected, washed with water, and 
dried as before. 

Chinese blue is mostly sold in the form of small cubical lumps, 
about 1 to 2 inches in diameter, but it is also sold in the form of 
fine powder. In grinding the blue great precaution is required 
to exclude particles of iron, as the production of a spark will 
ignite the dry powdered Chinese or Prussian blue and reduce it 
to a mass of red oxide of iron. 

A sample of Chinese blue examined by the author had the 
following composition : 

Water, ..... 4 "487 per cent. 
Oxide of iron, .... 52'055 ,, 
Cyanogen, .... 43 '508 ,, 

100-000 

PRUSSIAN BLUE. The commoner makes of blue are sold 
under the names of Prussian blue, Berlin blue, paste blue, <kc., 
in two forms, dry and pulp or paste. Some makes of these blues 
have a green shade, and others a violet shade or tint, owing to 
slight differences in the method of making. 

Green-tint Blues. There are several ways of making these 
blues. 1. Dissolving 1 cwt. each of yellow prussiate of potash 
and copperas in about 50 gallons of water, mixing the two solu- 
tions, allowing to settle, pouring off the clear top-liquor, washing 
the colour with water, then throwing it on to the filter and allow- 
ing it to be exposed to the air until it has acquired the desired 
blue ; to facilitate this the blue should be turned over from time 
to time so as to expose fresh surfaces to the action of the air. 



204: BLUE PIGMENTS. 

This method is not a good one as it leads to the production of 
oxide of iron in the colour, which affects the shade of the colour; 
it may be got rid of by treating the wet colour with hydrochloric 
acid, but this adds to the expense of making. 

2. The blue is precipitated as before, but the colour is de- 
veloped by adding bleaching powder. This process is identical 
with that used in making Chinese blue, but is less carefully 
carried out. 

3. Onecwt. of copperas, 201bs. of alum, and 10 Ibs. of sulphuric 
acid are dissolved in water and a solution of 1 cwt. of yellow prus- 
siate of potash added; the mixed solutions are allowed to stand for 
2 or 3 hours, when they are finished as in No. 1. The addition of 
alum makes the shade of blue lighter and, when dry, much easier 
to grind ; the proportion of alum added varies with different 
makers; at one time comparatively large quantities were added, 
but the tendency has been of late years to reduce the proportion. 
When a solution of yellow prussiate is added to one of alum 
there is no immediate precipitate, but on standing for about an 
hour a bluish-white precipitate falls down, the nature of which 
is somewhat uncertain. 

In the early days of Prussian-blue making blue-makers usually 
made their own prussiate, which, owing to the rough mode of 
preparation, yielded liquors containing a good many impurities, 
such as cyanides, sulphides, carbonates, and other salts of iron. 
The use of this crude liquor necessitated the use of much acid to 
prevent them from precipitating the iron in other forms than 
Prussian blue ; as the makers objected to the use of much acid 
they added alum instead, but, as a result, they got a paler blue. 
Blue makers rarely use such crude leys now, as the refined 
crystal prussiate is cheap and makes better blues with less trouble. 

Violet-tint Blues. These are sometimes known as Paris blue. 
They are made by dissolving 1 cwt. each of the red prussiate of 
potash and copperas in water, adding the two solutions together, 
allowing the precipitated blue to settle, pouring off the top- 
liquor, washing the residue with water, filtering, and drying the 
blue. 

When sold as pulp blues or paste blues the blue is simply 
allowed to drain on the filter and not dried ; such pulp colours 
contain from 25 to 30 per cent, of dry colour, as a general rule, 
although as little as 17 per cent, has been found in some makes; 
such pulp colours should always be bought under a guarantee of 
the quantity of actual dry colour they contain. 

SOLUBLE BLUE. While Chinese and Prussian blues are 
ordinarily insoluble in water and acids, yet a variety of the blue 



ANTWERP BLUE. 205 

can be made which is soluble in water ; so far as chemical 
composition is concerned this soluble blue does not differ from 
the insoluble varieties. Many recipes have been published for 
its preparation, of which the following are the principal : 

1. Prepare a solution of 100 Ibs. of perchloride of iron and 
10 Ibs. of Glauber's salt. A solution of 217 Ibs. of yellow 
prussiate of potash and 10 Ibs. of Glauber's salt is also prepared. 
The iron solution is poured into the prussiate solution, whereby 
a blue precipitate is obtained ; this is collected on a filter and 
washed with water until the wash-waters are tinged with blue ; 
it is then dried. In making soluble blue in this way it is 
important to pour the iron solution into the potash solution, 
and to keep the latter in excess. The object of adding the 
Glauber's salts is to ensure the complete precipitation of the blue 
by taking advantage of the fact that, while soluble in water, it 
is not soluble in saline solutions, so that by having the liquor 
saline there is a more complete precipitation of the blue. 

2. Dissolve 72 Ibs. of copperas in hot water, and pour this 
solution into a hot solution of 110 Ibs. of red prussiate of potash, 
and boil the mixture for two hours, filter, wash until the wash- 
waters have a blue colour, then dry the residual blue. 

3. Take 100 Ibs. of Prussian blue, mix well with about 
100 gallons of water, and add 50 Ibs. of yellow prussiate of 
potash, boil well for 3 to 4 hours, drain on a filter, wash as 
before, and dry. 

4. Dissolve separately in water 100 Ibs. of yellow prussiate of 
potash, and 80 Ibs. of copperas, add the two solutions together, 
and boil for 1 hour; then add 20 Ibs. of nitric acid and 10 Ibs. 
of sulphuric acid, and boil 1 hour longer ; then filter, wash, and 
dry as before. 

Soluble blue is not made on so large a scale now as formerly. 
It is used mostly for making blue ink and for painting velvets,, 
for which purposes it has been replaced by the aniline blues. 

ANTWERP BLUE. This blue has practically gone out of 
use, its place having been taken by the Brunswick blue described 
below. Antwerp blue is made as follows : 20 Ibs. of copperas, 
10 Ibs. of alum, and 10 Ibs. of zinc sulphate are dissolved in 50 
to 60 gallons of water, and to this solution is added one of 
40 Ibs. of the red or yellow prussiate of potash, dissolved in 50 
to 60 gallons of water. The blue is finished in the ordinary way. 

Antwerp blue is paler in colour than Prussian blue, and is 
probably a mixture of the ferrocyanides of iron, zinc, and 
alumina. Its properties are almost identical with those of 
Prussian blue. 



206 BLUE PIGMENTS. 

BRUNSWICK BLUE. Brunswick blue is a pigment of quite 
a recent origin, but it is now very largely made and used by 
painters. Essentially it is a mixture of Prussian blue and 
barytes, although some makers add other ingredients, as will be 
noted below. 

It is made in several shades deep, medium, and pale. The 
deep shade is made by thoroughly mixing 1 cwt. of barytes with 
50 to 60 gallons of water, adding a solution of 5 Ibs. of copperas, 
then a solution of 5 Ibs. of either the red or yellow prussiate of 
potash, taking care that the mass be kept well stirred during the 
whole time of mixing, so as to ensure the thorough incorporation 
of the barytes with the blue. After filtering, washing, and 
drying the blue is ready for use. For a medium shade the same 
quantity of barytes is used, but only 3 Ibs. of each of the other 
ingredients. The pale shade is made with 1 Ib. of the iron and 
potash salts to 1 cwt. of barytes. Some makers add a little 
ultramarine to the blue. 

Instead of barytes gypsum may be used, in which case the 
quantities of iron and potash salts must be increased by one-half, 
while china clay requires double the quantities to give blues of 
equal depth of colour. 

A sample of Brunswick blue analysed by the author had the 
following composition : 

Water, '275 per cent. 

Alumina, A1 2 O 3 2 '450 

Ferric oxide, Fe 2 3 , . . . . 3'310 ,, 

Barytes and silica, .... 89'860 
Cyanogen and sulphur, . . . 4 '105 ,, 

100-000 

This sample contained both Prussian blue and ultramarine, in 
the proportion of about 7 per cent, of the former and 5 per cent, 
of the latter. 

Brunswick blue is a very good pigment, is permanent, and not 
readily affected by exposure to air, light, &c. It is subject to 
one defect, viz., that when mixed with oil and turps into a paint 
the white portion is apt. on standing, to settle down, leaving the 
blue suspended in the liquid ; thus, in common parlance, the blue 
comes to the top \ this is easily remedied by giving the paint a 
stir before using. 

PROPERTIES OP PRUSSIAN BLUES. Prussian blues 
are characterised by their deep greenish-blue tint ; there is no 
other blue of the same depth of colour or of the same shade or 
tint. When dry, they have a bronzy appearance, which is 



PROPERTIES OF PRUSSIAN BLUES. 207 

greatest when the blue is pure, and is specially characteristic of 
the variety known as Chinese blue. 

They are very hard and exceedingly difficult to grind, and for 
Prussian blue to develop its full colouring powers it is essential 
that it be ground as fine as possible ; it is partly on account of 
this difficulty of grinding that Brunswick blue has come so largely 
into use, as painters are saved the trouble of grinding. 

Prussian blue is insoluble in dilute acids and, usually, in strong 
hydrochloric acid (see below) \ boiling with nitric acid turns it of 
a greenish shade ; while when boiled with strong sulphuric acid 
it is decomposed, sulphate of iron being formed, and hydrochloric 
acid, &c., evolved. Some samples are insoluble in strong hydro- 
chloric acid, others are soluble ; much depends upon the process 
used in making and also upon the age of the sample ; keeping 
seems to bring about some changes in the blue, whereby it is less 
easily soluble in the various agents which dissolve freshly- 
prepared blue readily enough. 

Oxalic acid dissolves Prussian blue very readily ; one part of 
oxalic acid will dissolve six parts of blue ; such solutions have 
been used to make blue inks, but since the aniline colours came 
into commerce the use of Prussian blue for ink-making has 
decreased. By digesting the ordinary Prussian blue with a satu- 
rated solution of oxalic acid at the ordinary temperature, it is 
slowly converted into the soluble variety ; on the other hand, if 
a solution in oxalic acid is boiled, the blue is gradually precipi- 
tated in the ordinary form ; the same result can be brought about 
by the addition of sulphuric acid. 

The most characteristic reaction of Prussian blue, which serves 
to distinguish it from other blues, is that when treated with any 
alkali, such as soda, potash, ammonia, or lime, it is decomposed 
into a ferrocyanide of the alkali and oxide of iron, the last being 
left behind as an insoluble residue having a red-brown colour, 
while the former passes into solution. Addition of acid in 
sufficient amount restores the blue colour, provided the ferro- 
cyanide of the alkali formed has not been washed away. The 
Prussian blues and the pigments containing it cannot be used 
with alkaline vehicles, such as lime, whiting, silicate of soda, or 
silicate of potash, because the blue would thereby be changed 
to red. 

As a pigment Prussian blue is quite permanent and resists 
exposure to air, light, and most of the other atmospheric in- 
fluences which act on pigments; it has, however, one curious 
property, that of fading a little on exposure to light and of 
recovering its original intensity of colour in the absence of light. 



208 BLUE PIGMENTS. 

Its colouring powers are very great, being by far the best of 
all the blues in this respect; it is rather a transparent colour, so 
that its covering power is not great. 

Heat decomposes it; the cyanogen it contains burns off as 
carbonic acid and nitrogen, while a blackish-brown or reddish- 
brown residue is left behind, according to the temperature to 
which it is exposed; a low temperature does not completely 
decompose it, and so a blackish-brown residue, consisting of a 
mixture of oxide of iron and carbon, is left behind ; on the other 
hand, a high temperature causes all the carbon to be burnt off 
and a reddish-brown residue of ferric oxide to be left behind. In 
some cases this last residue may be a mixture of the two oxides 
of iron, ferrous and ferric, or it may be a mixture of ferric oxide 
and metallic iron; the result depends upon the conditions under 
which blue is heated and its purity. The proportion of residue 
left depends upon the state of dryness of the blue, the process 
used in making it, and its purity; good qualities leave from 40 to 
50 per cent, of residue. 

Prussian blues can be mixed with nearly all other pigments 
without being affected or changed by them or affecting them in 
any way; the only exceptions are the few pigments of an alkaline 
character which destroy Prussian blue in the manner above 
described. 

ASSAY AND ANALYSIS OF PRUSSIAN BLUES. 
Prussian blues can be assayed for colouring power and tint by 
the usual methods. The paste blues should be assayed for the 
amount of actual colour they contain by taking a known weight 
and heating in a drying oven at from 120 to 130 F. for some 
hours; the loss in weight represents the amount of water present, 
and the difference is the amount of solid matter or dry colour. 

An analysis of Prussian blues is rarely required. When 
required, the blue should be boiled with ammonia until the blue 
colour is completely decomposed ; filter off the residue; the filtrate 
contains ammonium ferrocyanide and any zinc and magnesia 
which may be present. The two latter bodies can be tested for 
by the usual methods. The residue should be thoroughly well 
washed with water to free it from all traces of alkali; after which, 
it is treated with hydrochloric acid, when, if pure, it will all 
dissolve; if there is a white residue it may be barytes or gypsuni 
or china clay, which may be detected by the application of 
special tests. The solution will contain iron and alumina, and 
may be examined for these by the usual chemical methods. 

If the amount of Prussian blue in Brunswick blues (or other 
blues made by diluting Prussian blue with a white pigment) is 



COBALT BLUES. 209 

required, a known weight of the sample should be boiled in 
caustic soda until the blue is completely decomposed ; the mixture 
is then filtered and the brown residue well washed with hot 
water until it is quite free from alkali; next it is treated with 
hydrochloric acid until the brown oxide of iron has been dissolved; 
lastly, it is filtered, and to the filtrate is added ammonia in slight 
excess; the precipitate of oxide of iron thus formed is filtered off, 
washed with water, dried, and burnt in a crucible, and then 
weighed. The weight of the oxide of iron multiplied by 2-212 
gives the amount of Prussian blue in the sample taken. 

To detect Prussian blue in admixture with other pigments a 
portion should be boiled with caustic soda, when it will become 
brownish-red or change in shade, according to the character of 
the mixture and the amount of Prussian blue present. To 
further confirm the presence of Prussian blue the alkaline 
mixture should be filtered and to the filtrate as much hydro- 
chloric acid be added as will neutralise the alkali and then a 
little ferric chloride ; if Prussian blue be present, a blue precipi- 
tate will be obtained. 

COBALT BLUES. 

Cobalt is a metal characterised by the fine blue colour of some 
of its compounds, notably those with alumina, silica, and phos- 
phoric acid ; all these have a fine violet-blue tint, and, being very 
insoluble bodies and unaffected by most destructive agencies, 
they have been used to a greater or less extent as pigments. 

There are two commercial forms of cobalt blue. The one, 
smalts, which is really a glass coloured by cobalt, was largely 
used before it gave place to artificial ultramarine. The other, 
known as cobalt blue, is a compound of cobalt, alumina and, 
occasionally, phosphoric acid ; it has a fine pale blue tint and is 
mostly used by artists. 

SMALTS. Smalts has been used as a pigment for a very long 
period, although, of late years, its use has considerably declined 
owing to its tint being impaired by gas light, its weak colouring 
power, and its great inferiority to the much cheaper artificial 
ultramarine. 

Manufacture of Smalts. The manufacture of smalts takes 
place in three stages 1st, the preparation and roasting of the 
cobalt ore ; 2nd, the preparation of the blue glass ; 3rd, the 
grinding and levigation of the glass to form the smalts. 

1st Stage Preparation, &c., of the Ore. The ores of cobalt 
most used in the preparation of smalts are three in number viz., 



210 BLUE PIGMENTS. 

cobalt pyrites, Co 2 S s , so called because it resembles the common 
iron pyrites in appearance ; smaltine, Co As 2 ; and cobalt glance, 
Co 2 As 2 S 2 . These ores rarely occur quite pure, but are mostly 
found mixed with small quantities of other metals, such as iron, 
nickel, antimony, bismuth, and with earthy impurities ; these 
have to be separated before the ore can be used for the manu- 
facture of smalts, as many of them have an injurious effect on the 
tint of the finished article. This removal is effected in two ways 
1st, by hand-picking the ore; 2nd, by roasting the picked ore. 
The ore is carefully picked over by hand so as to remove the 
larger impurities ; the picked ore is next ground as fine as 
possible under edge-runners or stamps, and the lighter and more 
earthy particles removed by levigation, so as to leave a washed 
ore containing a large proportion of cobalt. This washed ore is 
then dried and placed in charges of about 3 to 5 cwts. on the 
hearth of a reverberatory furnace, where it is subjected to a 
slight roasting or oxidation. This reverberatory furnace is con- 
structed in a special manner ; the cobalt ores used contain a large 
amount of arsenic which is converted in the furnace into arsenious 
oxide. This oxide, being volatile, passes into the flues, which 
are arranged in long spirals round the furnace, so as to ensure 
its complete condensation, and thereby prevent any of this 
dangerous ingredient escaping into the air. The flues are cleaned 
out from time to time, for which purpose doors are made in the 
sides of the furnace communicating with different parts of the 
flue as shown in Fig. 22. This figure is a plan of a cobalt ore 
roasting furnace, where A is the bed or hearth of the furnace ; B 
the fireplace; C, 0, C, C are the various parts of the flue; 
D, D, D, D are doors communicating with the flue, through which 
the flue is cleaned out when required. The flue is only partly 
shown, as it winds round the furnace more than once. 

The roasting of the ore in this furnace causes the arsenic to 
become oxidised to arsenious oxide, As 2 O 3 , which is so completely 
volatilised that the roasted ore rarely contains any arsenic ; the 
sulphur in the ore is burnt to sulphur dioxide, S O 2 , which passes 
into the chimney ; while the metals are converted into oxides. 
This roasted ore is sold, under the name of zaffre, for colouring 
glass and pottery. For smalts-making it is found best not to 
roast the ore to the full extent, but to leave a little of the arsenic 
and sulphur in it ; the object of this is to ensure that in the next 
stage of smalts-making, all the impurities are removed. Cobalt 
has a great affinity for oxygen and but little for arsenic or sul- 
phur; whereas nickel and copper have great affinity for these 
elements ; so that when the roasted ore is melted with alkalies, 



SMALTS. 



211 



the copper and nickel with the iron, sulphur and arsenic combine 
together and form what is called a speiss or regulus, which collects 
under the glass ; when bismuth and antimony occur in the ore, 
they are partly liquated out before the actual roasting, and the 
rest passes into the regulus. 

The percentage of cobalt in the roasted ore varies so con- 




Fig. 22. Cobalt ore roasting furnace. 

siderably that no definite instructions for smalts-making can be 
given in text-books; the quantities of materials used depend 
upon the strength of the zaffre, as will be noted below. 

2nd Stage Preparation of the Blue Glass. The roasted ore is 
now mixed with silica and potash and melted in a furnace, when 
the cobalt, silica, and potash combine together to form a blue 
glass. The potash used must be of the best quality and as free 
from impurities as possible; soda, in particular, should be avoided, 
as its influence on the tint is bad; iron and other metals should 
also be absent. The silica is used in the form of quartz, and is 
purified by hand picking, so as to have it as free as possible from 
lime, alumina, and ferruginous impurities, as these cause the 
tint of the glass to be very dull. The quartz is ground as finely 
as possible under edge-runners, and the various ingredients are 
carefully and thoroughly mixed together in wooden tubs, the use 
of metal being avoided for fear of any getting among the mixture. 



212 BLUE PIGMENTS. 

Sometimes a little white arsenic is added so as to ensure that 
any iron which may be in the mixture shall find its way into 
the regulus. 

The proportions of zaffre, silica, and potash used varies accord- 
ing to the strength of the zaffre (which is the actual colouring 
agent) and to the depth of colour required in the finished pro- 
duct; usually the amount of potash is kept at a fixed quantity 
viz., one-third the total weight of the ore and quartz combined. 
The actual proportions used are always determined by a trial on 
a small scale with every fresh batch of zaffre, and from the depth 
of colour of the glass thus obtained the manufacturer judges as 
to the proportions needed on the large scale to produce the shade 
of smalts required . The various ingredients are carefully weighed 
out and thoroughly mixed (as noted above); too much care 
cannot be taken on this point, as upon it depends much of the 
success of the operation. 

The materials are now placed in earthen crucibles made of as 
strongly a refractive clay as can be obtained; these crucibles 
must not contain any lime. They are usually about 18 inches in 
diameter at the top and about 14 inches at the bottom; they hold 
about 84 Ibs. of material. They last for from six to seven months. 

The furnace in which these pots are heated is similar in con- 
struction to a glass furnace, although other forms may be used. 

A German smalts furnace is shown in Fig. 23, where A A are 
two of the pots, and B an opening through which the pots are 
placed in the furnace; there are several of these openings placed 
round the furnace. After the pots have been placed in position 
the openings are bricked up, a hole being left for the purpose of 
removing the speiss, the glass, &c. These openings are kept 
closed up during the time the melting is proceeding. Above the 
openings, B, are small ones through which the materials in the 
crucible are worked and through which the blue glass is removed 
at the end of the operation. E is the fireplace. F F are flues in 
the roof of the furnace communicating with the chimneys, G G. 

The pots, after they have been placed in the furnace and all 
openings (except the working ones) have been closed up, are 
charged with material by means of iron ladles, and the fire 
lighted. In about 8 hours the material begins to fuse or melt ; 
it is then thoroughly stirred and the temperature raised to a 
white heat, when the glass begins to form. When the mass has 
become quite homogeneous (which is ascertained by taking small 
samples from time to time from the pots) and it is seen that the 
speiss has settled out, the glass is ladled out into cold water and 
the speiss drawn off from the bottom of the pots by the holes 



SMALTS. 



213 



which have been made for that purpose; after which the pots are 
re-charged for another lot, which is done through the hole at 
the top of the furnace-opening, B. The object of throwing the 
glass into cold water is to break it up into small pieces so as to 
facilitate the grinding. 

3rd Stage Grinding. The blue glass thus obtained is in 
angular fragments of about an inch in size, which are ground 
and levigated to form the smalts. The grinding is done very 
thoroughly, so as to have the material as fine as possible. In 
some works the first or rough grinding is done under stamps ; 
then the rough material is further ground to the required degree 




Fig. 23. Smalts furnace. 

of fineness under edge-runners with water ; or, if required, the 
whole grinding may be done with a set of edge-runner mills. 
After some hours grinding, the mass of ground material is sent 
into a series of settling tanks, in the first of which the coarsest 
particles settle, but, being too coarse for use, these are sent 
through the edge-runners again ; in the next tanks the smalts 
collects ; this is taken out of the tanks, dried, and sent into the 
market. The fine material which settles out in the last tank of 
the series is usually of too pale a colour to be of use, and the 
maker generally returns it to the crucible to be remelted. 



214 BLUE PIGMENTS. 

Five tons of glass yields about 3 tons of smalts ; the loss of 2 tons 
is partly due to the rejection of the coarse and fine material, 
which, however, is not actually lost, as it finds its way back in 
subsequent lots ; the actual loss arises from some of the material 
being carried away in the wash-water, partly in suspension and 
partly in solution. 

COMPOSITION AND PROPERTIES OP SMALTS. 
Smalts is a pigment of a violet-blue shade by daylight which 
changes to a reddish-blue by gaslight ; its colouring power is 
very weak, and hence it is not much used. The greatest con- 
sumers were bleachers, but its use for this purpose has much 
declined of late years. The tint of smalts varies from time to 
time, somewhat according to the proportions of the ingredients 
which have been used in its manufacture, and to the purpose for 
which it is used ; in fineness of powder it also varies a little, and 
the depth of colour depends largely upon this feature ; the finer 
the powder, the paler the shade or tint of the smalts. It mixes 
with both oil and water, but in neither case does it make a good 
pigment for water- or oil-painting. 

In composition it is essentially a cobalt glass a double silicate 
of potash and cobalt, with a few minor impurities ; Rivot gives 
the following analysis of a sample of smalts : 

Silica, Si 2 , ..... 56 '4 per cent. 

Alumina, A1 2 3 , 3 '5 

Ferric oxide, Fe 2 O 3 , . . . . 4'1 

Cobalt oxide, Co 0, . . . 16 '0 

Calcium oxide, Ca 0, . . . . 1 '6 

Potash, K 2 0, 13-2 

Lead oxide, Pb 0, .... 47 

99-5 

The composition of smalts varies very much, especially in the 
amount of the impurities. 

It is not acted upon readily by acids or alkalies, which two 
points differentiate smalts from other blues ; by these and its 
tint it can be readily distinguished. As a pigment it is per- 
manent, perfectly resisting exposure to light, air, &c. It can be 
mixed with all other pigments without affecting them or being 
affected by them. 

Smalts is sold in a variety of tints and qualities, distinguished 
by marks, to which some makers attach a meaning, such as FC, 
fine colour; FOB, fine colour Bohemian; FE, fine eschel; MC, 
medium colour; OC, ordinary colour, &c. These marks vary with 
different makers and represent no standard in quality or tint. 



COBALT BLUE. 215 

ASSAY AND ANALYSIS OF SMALTS. Smalts requires 
assaying for its tint or colour, colouring power, and fineness of 
powder. 

As smalts is rarely, if ever, adulterated, an analysis is rarely 
required, either qualitative or quantitative. The action of dilute 
acids will soon show whether a sample of smalts has been adul- 
terated with ultramarine or some of the copper blues, while the 
action of alkalies will soon distinguish the addition of Prussian 
blue. Smalts is absolutely unaffected by these two reagents. 

Long digestion with strong hydrochloric acid decomposes it, 
leaving a residue of gelatinous silica and forming a solution of 
the chlorides of the metals it contains. 

COBALT BLUE. 

This blue is much used by artists on account of the purity of 
its tint and its permanency. 

PREPARATION OF COBALT BLUE. Cobalt blue is 
essentially a compound of the oxides of cobalt and alumina ; 
some makers add phosphoric acid in making it; there is some 
small advantage in doing so, the tint being a little finer. 

One method of making cobalt blue is to mix together solutions 
of alum and cobalt in the proportion of 1 Ib. of cobalt nitrate to 
12 Ibs. of alum ; to the solution of these two bodies sufficient 
carbonate of soda is added to completely precipitate them; the 
precipitate is collected, placed in a crucible, and heated to a red 
heat ; when the blue colour has properly developed, the mass is 
washed with water, dried, and ground up for use. 

The best method of making it is to dissolve nitrate of cobalt in 
water, and to add to the solution sufficient sodium phosphate to 
precipitate all the cobalt as phosphate of cobalt ; this precipitate 
is collected on a filter, and well washed with water. A solution 
of alum or alumina sulphate is precipitated with one of sodium 
carbonate, the precipitate is collected and washed with water. 
The two precipitates are now mixed together in the proportion 
of 8 parts of the alumina to 1 of the cobalt phosphate precipitate, 
and the mixture is heated to a bright red heat in a crucible for 
from half to three-quarters of an hour ; when the blue has fully 
developed, the mass is ground with water and dried, after which 
it is ready for use. 

PROPERTIES AND COMPOSITION OF COBALT 
BLUE. Cobalt blue has a greenish-blue tint, which is very 
fine, although it has a tendency to become of a slightly violet 
tint under the influence of gaslight. It is quite permanent 



216 BLUE PIGMENTS. 

when exposed to light and air, and hence is largely used by 
artists, especially water-colour artists, as it works better in 
water than in oil. Cobalt blue can be mixed with all other 
pigments without affecting them in any way, or being altered 
itself; this is a feature of some considerable importance. It 
is unaffected by treatment with either acids or alkalies. 

It is a compound of the oxides of alumina and cobalt, with, 
occasionally, some phosphoric acid. The following analysis of a 
sample, made by the author, will serve to show the average 
composition of cobalt blue : 

Water, . . . . . 3 '075 per cent. 

Alumina, A1 2 3 , .... 80'795 ,, 

Cobalt oxide, Co 15'132 

Alkaline salts, -998 

100-000 

ASSAY AND ANALYSIS OF COBALT BLUE. Cobalt 
blue is rarely subjected to any tests for purity or other pro- 
perties. Its tint, colouring power, &c., can be examined by the 
usual methods. Its tint and unalterability by treatment with 
alkalies and acids serves to distinguish cobalt blue from other 
blues. 

By strongly heating cobalt blue with strong sulphuric acid for 
some time it is decomposed, a violet solution and a white powder 
being obtained ; on diluting with water, the latter dissolves, and 
a clear blue solution is obtained, which can be examined by the 
usual methods of metal analysis. 

Cobalt blue has been sold under a variety of names, such as 
Gahn's ultramarine, Thenard's blue, cobalt ultramarine, azure 
blue, &c. 

COPPER BLUES. 

There are a few blue pigments which owe their colour to 
copper. At one time some of these were very largely used for 
painting of all kinds, but they have had to give place to ultra- 
marine, which is at once a more powerful colour and more 
permanent. They are not expensive colours, but are only used 
now to a very limited extent by artists, and scarcely, if at all, by 
house-painters. The copper blues are known under a variety of 
names : mountain blue, Bremen blue, lime blue, blue verditer, 
&c. All these blues have a similar composition and very similar 
properties. 

MOUNTAIN BLUE. This blue pigment (the azurite of 



BREMEN BLUE. 217 

mineralogists) is found naturally, and is essentially a basic 
carbonate of copper, having the composition : 

Copper oxide, Cu 0, .... 69 2 per cent. 
Carbonic acid, C 2 , . . . . 25 '6 
Water, 5'2 

100-0 

which corresponds to the formula 2 Cu C O 3 + Cu H 2 O 2 . 

For use as a pigment the mineral is ground up very fine; it is 
of a fine tint of blue, and is much more permanent than any of 
the other copper blues. It is not much used as a pigment. 

BREMEN" BLUE. Bremen blue is a pigment of a pale 
greenish-blue tint without much colouring power ; at one time 
it was made on a large scale, but is now replaced by ultramarine 
and cobalt blue, so that now it is only made on a limited scale. 
There are several ways by which it can be made. 

L 125^ Ibs. of common salt and 111 Ibs. of copper sulphate are 
ground together into a paste, with water; this results in the 
formation of chloride of copper and sulphate of soda. With the 
paste is mixed about 1 cwt. of clean copper, in small pieces 
about 1 cubic inch in size. All these are thoroughly mixed to- 
gether and kept in wooden boxes or tubs ; at intervals of two or 
three days the mass is turned over with a wooden spade, so as to 
ensure that the metal and the paste are brought into intimate 
contact with one another. In about three months all the copper 
will have been converted into a green basic oxychloride of 
copper, which was at one time sent out as a pigment under the 
name of Brunswick green; this basic oxychloride is insoluble in 
water, and, after it has been formed, the mass is thrown into 
tubs and thoroughly washed with water, by which means all the 
soluble alkaline compounds are washed out. The green is now 
ready for being converted into the blue; to effect this the green 
is mixed with a small quantity of hydrochloric acid and allowed 
to stand for 24 hours ; to the pasty mass is then added about 
2^ times its volume of caustic soda, at 40 Tw., which is 
thoroughly mixed with it ; and then the mass is allowed to stand 
for 36 to 48 hours, by which time it will have been converted 
into the required blue ; it is now thoroughly washed with water 
to free it from soda, and dried, when it is ready for use. 

2. 50 Ibs. of sulphate of copper and 26 Ibs. of common salt are 
dissolved with a small quantity of water, heat being used to 
facilitate the operation. The solution is gradually poured into a 
solution of 50 Ibs. of soda crystals, when a precipitate of copper 



218 BLUE PIGMENTS. 

carbonate is formed ; on allowing this to stand for some time 
it is gradually changed into basic chloride of copper, and as 
a considerable effervescence occurs, owing to the liberation of car- 
bonic acid, large vessels must be used. After being washed, the 
green chloride is transformed into the required blue by the same 
process as described above. 

3. A cheap method of making Bremen blue consists in making 
a solution of sulphate of copper, to which is added a solution of 
chloride of calcium or of chloride of barium as long as a white 
precipitate falls; this is allowed to settle, and the clear blue 
liquor obtained is mixed with a quantity of freshly-prepared milk 
of lime until all the copper has been precipitated. This is known 
by allowing the precipitate to settle and noting the colour of the 
liquor; if this is blue, then more lime is required. As a rule, 
20 Ibs. of quicklime is sufficient, when converted into milk of 
lime, to form 100 Ibs. of sulphate of copper. The precipitate is 
allowed to settle, washed, and dried ; while the clear liquor, 
which is a solution of chloride of calcium, may be used to pre- 
cipitate fresh sulphate of copper. The results are not so good as 
with the methods described above. 

Bremen blue consists mostly of hydroxide of copper, Cu H 2 2 , 
with small quantities of carbonate of copper. 

BLUE VERDITER. Blue verditer is a pigment of a sky- 
blue tint. It is very similar to Bremen blue in its composition 
and mode of preparation. 

1. A solution of copper sulphate of 1-312 (62 J Tw.) specific 
gravity is prepared and heated, and a hot solution of calcium 
chloride added until no further precipitate is obtained. The mix- 
ture is filtered, and the liquor, which consists of a solution of 
copper chloride, is diluted with water until it has a specific 
gravity of 1'157. Slaked lime is thoroughly ground with water 
to a great degree of fineness, and added to the copper solution in 
small quantities at a time, until all the copper has been precipi- 
tated. The mixture is now filtered, drained, and washed, and a 
small portion of the paste weighed and dried as rapidly as possible 
to ascertain the amount of actual dry colour it contains. The 
green paste thus obtained is placed in wooden tubs, and for every 
35 Ibs. of dry colour it contains, 4 Ibs. of the lime paste, made 
as above described, and 2J pints of a solution of carbonate of 
potash of 1-116 (25 Tw.) specific gravity is added, and thor- 
oughly stirred with it. The mass is allowed to stand, and, when 
the proper shade has been developed, it is washed with water, 
filtered, and dried, when it is ready for use. 

2. Another method, which is really an extension of the last 



LIME BLUE. 219 

method, is used in some works in Germany. The process is, up 
to the stage of producing the green paste and mixing it with 
lime and carbonate of potash, identical with the last method ; 
but, now, the paste is placed in vessels which can be hermetically 
sealed, and to every 35 Ibs. of dry colour there is added a 
solution of 1 Ib. of ammonium chloride, and 2 Ibs. of copper 
sulphate, in 3 J gallons of water ; when all are mixed together, 
as thoroughly as possible, the vessels are closed up, and left for 
four to five days, after which they are opened, and the colour 
washed and dried in the usual way. 

3. A solution of copper sulphate or nitrate is prepared, and to 
it is added a solution of either the carbonate of potash or the 
carbonate of soda as long as a precipitate falls down ; this is 
collected and washed, then treated with a weak solution of 
caustic soda to turn it blue (as is done in making Bremen blue). 

Any copper solution may be used, but the nitrate or chloride 
gives the best results. 

What is called "refiners' blue verditer" is prepared from the 
copper solution obtained in refining gold or silver. It differs in 
no way from the verditer prepared from other copper solutions. 

A sample* of refiners' blue verditer, examined by the author, 
had the following composition : 

Copper carbonate, Cu C 3 , . . . 77 '797 per cent. 

Copper sulphate, Cu S 4 , . . . . 9 '426 ,, 

Copper oxide, Cu 0, 12'350 ,, 

Water, hygroscopic, . . . . . '775 ,, 

100-348 
Another sample of blue verditer had the composition : 

Copper carbonate, Cu C 3 , . . . 62 '45 per cent. 

Copper hydroxide, Cu H 2 2 , . . . 31 '19 ,, 

Water, hygroscopic, 3 '29 

Calcium sulphate, Ca S 4 , . . . 3 '07 

100-00 

Blue verditer is used to a small extent by artists, especially in 
water-colours ; but it is not a permanent pigment. 

LIME BLUE. Before the introduction of artificial ultra- 
marine lime blue was very largely used for fresco-painting, and 
common lime-washing and distemper work, on account of its 
resisting the action of alkaline vehicles and pigments. It has, 
however, gone out of use, so much so that it is extremely difficult 
to procure a genuine sample of lime blue, ultramarine being sold 
for it under the name of lime blue. 



220 BLUE PIGMENTS. 

Lime blue can be made by the following methods : 

1. 125 Ibs. of copper sulphate are dissolved in water, and to 
the solution is added 12 J Ibs. of sal-ammoniac dissolved in warm 
water; 30 Ibs. of good clean quicklime are carefully slaked with 
water, and the slaked lime ground into a fine paste with water, 
after which it is made into a milk by adding more water. The 
milk of lime is poured into the copper solution, both being well 
mixed by constant stirring; when all the lime has been added 
a blue precipitate and a blue solution will be obtained; this 
mixture is allowed to stand until the solution has become 
colourless, taking care to stir it up from time to time while the 
decoloration is proceeding. The blue pigment formed is filtered, 
washed with water, and dried. 

2. A strong solution of sulphate of copper is prepared, and to 
this is added sufficient ammonia to redissolve the precipitate 
first obtained. The solution is heated slightly, and to it is added 
milk of lime, prepared as in the preceding method ; the blue 
gradually precipitates, and is collected, washed, and dried. 

Lime blue is essentially a mixture of copper hydroxide and 
calcium sulphate. 

In preparing all these copper blues care must be taken not to 
have the liquors too hot, as, if so, there is a liability for the 
copper precipitate to be decomposed and the black oxide of 
copper to be formed. 

PROPERTIES OF THE COPPER BLUES. As the pro- 
perties of the copper blues are so similar a general description 
will suffice. All the copper blues are characterised by being of 
a pale greenish-blue tint, varying a little in shade and colouring 
power; being opaque, they are good pigments, especially in water, 
but in oil they lose some of their opacity. Although not quite 
permanent, yet they resist a considerable amount of exposure to 
light and air; they are blackened by sulphuretted hydrogen or 
by sulphur, owing to the formation of the black sulphide of 
copper; on this account they cannot be used in places where 
they are likely to come in contact with sulphur or sulphur gases, 
nor can they be mixed with other pigments containing sulphur. 
Exposed to heat they blacken, owing to the loss of water and of 
carbonic acid and the formation of the black oxide of copper. 
Acids dissolve them, forming blue solutions which give the 
characteristic tests for copper, such as the deep blue solution 
with ammonia and the brown precipitate with ferrocyanide of 
potassium. Alkalies have little action in the cold; but when 
heated with them they turn black, owing to the formation of 
the black oxide of copper. 



OERULEUM. 221 

Ammonia usually has a solvent action on the copper blues, 
forming a deep blue solution; by the tests here given copper 
blues can be readily distinguished from other blues. 

The copper blues are rarely met with, their place having been 
taken by ultramarine and cobalt blue. 



C-33RULEUM. 

Under the name of Cseruleum two pigments are known; one of 
these is, or rather was, an old one used by the ancient Egyptians 
in the decorations of their temples and tombs; this pigment was 
a very permanent one and of a fine tint of blue, but nothing is 
now known as to how it was made, although, lately, a French 
chemist has announced that he has made this colour in all its 
original properties. 

The other pigment is a more modern one, of a fine tint, but of 
no great permanence. The two pigments will now be described 
as fully as is necessary. 

C^JRULEUM. The fine blue pigment found on the paintings 
and decorations of the ancient temples of Pompeii, Alexandria, 
Cairo, and other old cities testifies how largely it was used by the 
ancient Egyptians, who were, in all probability, the discoverers 
of it. So far as can be discerned, the paintings on these ancient 
temples are of as bright a blue colour now as on the day when 
they were painted, although they have been exposed to the 
weather for more than 1,000 years. There is no trace of its 
manufacture and use after the barbarian invasion of Italy. 

The blue has been examined by Chaptal, Sir Humphrey Davy, 
Girardin, and others, but without any result, so far as its pre- 
paration was concerned; and, yet, if it could be produced at a 
low price it would be very extensively used by all classes of 
painters. 

A. more recent observer, Fouque, has examined this pigment, 
and in a memoir communicated to the Academy of Sciences of 
Paris, and published in the Comptes Rendus, 1887, pp. 108, 325, 
he describes the results of his analyses of the blue, and of his 
experiments for its production. Fouque gives the composition 
of this old pigment as 

Silica, Si 2 , 63- 7 per cent. 

Calcium oxide, Ca 0, . . . . 14 '3 ,, 

Copper oxide, Cu 0, . . . . 21*3 ,, 
Ferric oxide, Fe 2 O 3 , . . . . 0'6 

99-9 



222 BLUE PIGMENTS. 

He considers it to be a double silicate of copper and calcium, 
having the formula 4 Si O 2 , Ca O Cu O; the iron being, as might 
be expected from its small amount, an accidental impurity. The 
ancients, he considers, obtained it by fusing together roasted 
copper ore with sand and lime. 

Fouque states that he has obtained a blue colour of a similar 
composition in a crystalline condition ; the crystals were dichroic, 
appearing of a deep sky-blue tint when seen by surface reflection, 
and of a pale rose tint when seen through the edges. There are 
a few practical difficulties in its preparation, which consist in 
heating to a bright red heat a mixture of its component parts ; if 
the heat becomes too high, then the blue colour disappears and 
only a green coloured glass is obtained. The blue pigment thus 
obtained is said to be quite permanent; it remains unaffected 
when boiled with sulphuric acid, soda, or lime, or any other 
alkali or acid ; and it is unaffected by sulphuretted hydrogen. 

Peligot's Blue is a similar preparation made by fusing together 
73 parts of silica, 16 parts of copper oxide, 8 parts of lirne, and 
3 parts of soda; the temperature must not be allowed to exceed 
about 800 F., or the colour will be changed from a blue to a 
black ; if the soda was omitted in this process, the results would 
be nearer those of the original blue. Peligot's blue has not come 
into use. 

C-33RULETJM (2). Messrs. Rowney & Co. have offered to 
artists a fine light blue of a greenish tone, for which they adopt 
the name cseruleum ; it is a compound of the oxides of tin and 
cobalt and is a fairly permanent pigment. The method of pro- 
ducing it has never been published, but, possibly, it is made by 
preparing a solution of stannate of soda and precipitating this 
with a solution of cobalt nitrate ; the precipitate will consist of a 
mixture of the oxides of cobalt and tin ; the precipitate is heated 
to a bright red heat, when the blue pigment will result. Another 
method is to mix together solutions of tin and cobalt, and pre- 
cipitate with soda ; after washing free from the alkali, the pre- 
cipitate is heated as before. By using silicate of soda as the 
precipitating agent, so as to obtain a precipitate containing 
silica, tin, arid cobalt, a fine blue could be obtained. 

Several other bodies of a blue colour have been suggested for 
use as pigments, but, partly on account of their greater expense, 
they have not been able to compete with ultramarine or Prussian 
blue as pigments ; hence, their use has been either limited or 
abandoned. One may be mentioned. 

Manganese Blue. Some years ago Bong gave a description 
of the process of making manganese blue, which resembles very 



C^EULEUM. 223 

much that of ultramarine. He gives the following mixtures 
which may be used : (1) 3 parts of silica, 6 parts of soda ash, 
5 parts of calcium carbonate, and 3 parts of manganese oxide. 
(2) 3 parts of silica, 3 parts of manganese oxide, and 8 parts of 
barium nitrate. (3) 2 parts of kaolin, 3 parts of manganese 
oxide, and 8 parts of barium nitrate. 'In each case the mixture 
is heated to a red heat in an oxidising atmosphere. Iron must 
not be present in the ingredients. By varying the proportion of 
manganese the intensity of the blue can be varied, but not its 
tint ; on the other hand, by increasing the proportion of alkali 
and silica the blue becomes more violet or green. 




224 



CHAPTER VII. 
BROWN PIGMENTS. 

THIS group of pigments is a small one umber, vandyke-brown, 
sepia, manganese brown, Cappagh brown, and one or two others 
of little importance complete the list. Most of these browns are 
natural pigments. 

TIMBER. 

Probably the most important of the brown pigments is umber. 
It is an earthy pigment closely resembling the ochres and siennas 
in its composition and properties; in fact, these three pigments 
form a natural group of yellow to brown colours having the 
ochres at one end of the scale and the umbers at the other, while 
between the two extremes it is possible to find all or nearly all 
the intermediate tints or shades. 

Umber is found native in many places; as in Derbyshire, 
Devonshire, Cornwall, Wales, &c., in this country; in France; in 
Italy; and in many localities in America. The finest umber 
comes from Cyprus. 

It is found in veins and layers of varying thickness in rocks 
of all geological ages, and from which it has, in some cases, been 
derived by decomposition. At Ashburton, near Dartmoor, the 
umber is found in a layer of from 20 to 30 feet in thickness, 
overlying the bed rock, which is a dolomitic limestone contain- 
ing some manganese and iron and from which it has been formed; 
above the umber there is an overburden of soil. 

Umber varies somewhat in hue from a reddish-brown to a 
violet-brown, the former hue being characteristic of the Derby- 
shire umbers, while Turkey umber (which comes from Cyprus) 
has a warm violet-brown hue. This umber owes its trade name 
to the fact that it was imported through Constantinople, and its 
real source was at one time not properly known. 

Umber is sold in three forms raw lump umber, raw powdered 
umber, and burnt umber. 

Raw lump umber is the pigment just as it is obtained from the 



COMPOSITION AND PROPERTIES OF UMBERS. 



225 



mines. The method of mining will vary somewhat according to 
the varied conditions under which the umber is found. At Ash- 
burton it is mined by taking off the overburden of soil, digging 
square pits until the bed rock is reached, and lining the pits with 
timber as the material is removed. When the bottom is reached 
these timbers are removed and fresh pits sunk. A similar method 
is probably in use for mining other deposits of umber. 

Powdered raw umber is the lump umber ground and levigated 
in the same manner as ochres are treated. 

Burnt umbers are the raw umbers calcined at a red heat in a 
furnace, by which treatment the colour becomes darker and 
warmer; the change which occurs is similar to that which ensues 
when ochres and siennas are calcined (see pp. 105, 136, 141). 

COMPOSITION AND PROPERTIES OF UMBERS. 
Umbers have, as has already been stated, a great resemblance to 
the ochres and siennas in their composition ; but they contain 
more manganese, which probably accounts for their darker 
colour. The following analyses of various colours will serve to 
show the average composition of these pigments : 





1. 


2. 


3. 


4. 


Water, hygroscopic, . . . 
Water, combined, .... 
Silica, Si 2 , 


4-325 
8-450 
29 566 


13-475 
5-175 
4-430 


6-612 
10250 
24'518 


65-00 
4-80 
12-30 


Calcium carbonate, Ca C 0$, 
Manganese, Mn 2 , . . . 
Alumina, A1 2 3 , .... 
Ferric oxide, Fe 2 3 , . . . 
Barium sulphate, Ba S 4 , . 
Calcium sulphate, Ca S 4 , . 


5-560 
12-285 
2-735 
36-475 


2-607 
11-530 
8-078 
22-500 
30-105 
2-153 
trace 


6-090 
7'110 
12-802 
29-983 

2 : "l37 


io : 6o 

6 : 30 


Lime, Ca 0, . . ... 








60 










1-00 














99-394 


100-053 


99502 


100-00 



No. 1 is an analysis of Cyprus umber ; this sample had a dark, 
warm brown tint. 

No. 2 is an analysis of crude Derbyshire ochre ; this sample 
was of a soft character and of a reddish -brown tint. 

No. 3 is an analysis of a sample of umber, probably of English 
origin. This sample had a yellowish-brown tint. The above 
analyses were made by the author. 

15 



226 BROWN PIGMENTS. 

No. 4 is an analysis made by J. J. Beringer (quoted by 
Frecheville*) of the umber from Ashburton, already referred to ; 
and the next shows the composition of the dolomitic limestone 
from which it is derived. 



Water, hygroscopic, 
Calcium carbonate, Ca C 3 , 
Magnesium carbonate, Mg C 63, 
Silica, Si O 2 , . . ' . 
Ferrous carbonate, Fe C O s , 
Manganese carbonate, Mn C (>3, . 




0'8 per 
46-7 
41-0 
3-3 
2-6 
2-5 
3-1 


cent. 



100-0 

The decomposition has probably been brought about by the 
dissolving action of carbonated water on the calcium and 
magnesium carbonates, so as to leave the iron, manganese, and 
silica behind to form the umber; the amount of limestone which 
must have been disintegrated to form deposits of umber, 20 to 
30 feet in thickness, must have been enormous, as but little of 
the calcium and magnesium remains behind in the umber. The 
umber found at Yeryan (near Truro), Milton Abbot, and other 
places in Cornwall is undoubtedly formed from limestone rocks 
in a similar manner. 

Umbers are pigments of a warm brown colour, varying in hue 
from yellowish to violet-brown. By calcining, the colour is 
rendered darker and warmer. As pigments, they work well 
in both oil and water, and they can be mixed with all other 
pigments without any change occurring. They are perfectly 
permanent, being unaffected by all the ordinary conditions to 
which pigments are exposed. Umbers, therefore, meet with 
extensive use among all classes of painters. 

Umbers are not readily attacked by acids, but prolonged 
digestion with strong hydrochloric acid dissolves the larger 
proportion of the umber, forming a brownish-yellow solution 
containing iron, alumina, manganese, and lime; the silica and 
the barium sulphate remain undissolved. The metals may 
be tested for by the usual analytical methods. Caustic soda 
has no action on umbers. 

ASSAY AND ANALYSIS OF UMBERS. Umbers may 
be assayed for colour or hue, colouring power, covering power, 
and similar properties by the usual methods. 

A chemical analysis of umbers is rarely required, as they are 
rarely, if ever, adulterated, except possibly a dearer umber by a 

* Trans. Roy. Oeolog. Soc., Cornwall, xvii., p. 217. 



VANDYKE-BROWN. 227 

cheaper one; but this kind of adulteration would be very difficult 
to detect. In case an analysis is required the method detailed 
under oxide reds (p. 108) is applicable here. 



VANDYKE-BROWN. 

Next to umber the most important brown pigment is Vandyke- 
brown, so named after the great painter, who was particularly 
partial to the use of browns in his pictures. No record remains 
as to the origin of the particular brown which he used, but, it 
was presumably a natural brown found, perhaps, in the vicinity 
of the artist's residence and, probably, more or less organic in its 
origin. The Vandyke-browns now sold are, however, all of arti- 
ficial production from a variety of sources. In many works on 
pigments it is stated that this pigment is prepared by calcining 
ochres and copperas ; it is extremely doubtful whether these 
browns were ever made in this way, as neither ochres nor cop- 
peras yield Vandyke-browns when calcined. 

Vandyke-browns are made in several ways : 

1. From natural deposits of a brown colour occurring much in 
the same manner as the ochres and umbers, but differing from 
them in being derived largely from organic sources, such as peaty 
matter mixed with more or less earthy matter. Sometimes these 
pigments are named after the localities in which they are found, 
as, for instance, Cassel earth. 

For use as pigments these natural products simply require to 
be ground as fine as possible. 

2. From cork cuttings and waste, bark and twigs of trees, and 
other organic matter of vegetable origin, by calcining slightly in 
a closed vessel. 

These Vandyke-browns have a warm brown colour of a reddish 
hue ; they mix very well with oil and water, and can be used for 
all kinds of painting. 

An analysis of such a brown made by the author showed it to 
have the following composition : 

Organic matter and water, . . . 70*289 per cent. 

Calcium carbonate, CaCO 8 , . . . 3 '490 ,, 

Oxide of iron and alumina, . . . 1'615 ,, 

Alkaline carbonates and alkaline salts, . 24 '606 



100-000 



3. Most of the common Vandyke-browns are made by mixing 
together lamp-black, vegetable black, or other black pigment with 



228 BROWN PIGMENTS. 

red oxide and a little yellow ochre ; the proportions used vary 
according to the quality and shade of the oxide used, and whether 
ochre is also used. Vandyke-browns thus made, and containing 
from 36 to 50 per cent, of black (chiefly lamp-black) form the 
great bulk of these pigments as used by the house-painter. This 
variety of Vandyke-brown is a permanent pigment, and works 
well in oil, if care be taken in regard to the quality of the black 
used ; if it has any fault, it is that of being a bad drier. In 
water it does not mix quite so readily as the other varieties of 
Vandyke-brown, although, when mixed, it works well. 

Vandyke-brown is sold in the form of small angular pieces, 
powder, and paste ground with either oil or water according to 
the use it is to be put to. 

PROPERTIES OP VANDYKE -BROWN. Vandyke- 
brown is a perfectly permanent pigment and withstands any 
amount of exposure to light and air. It works well in either oil 
or water and with any kind of vehicle. It can be mixed with all 
other pigments without any alteration whatever. 

SEPIA. 

Sepia is a brown pigment of slightly varying hue, much used 
by artists, especially for monochrome work. It is obtained from 
various species of cephalopodous animals, such as Sepia officinalis, 
Sepia loligo, &c. These animals have a peculiar gland which 
secretes a blackish-brown liquor that collects in what is called 
the ink-bag. This liquor is secreted for the purpose of defence; 
when an enemy approaches, some of the contents of the bag is 
discharged, and this, owing to its strong colouring powers, 
colours the water for some distance around, and under the cover 
of the opacity thus produced the sepia makes its escape. The 
animals are caught and the ink-bag carefully taken out and 
dried ; it then forms the pigment known as sepia. The com- 
mercial article is in the form of small pear-shaped pieces of a 
blackish-brown colour, to which fragments of the sac or skin of 
the bag usually remain attached. For purposes of use as a 
pigment for artists it is necessary to remove the sac or bag; 
this is done by boiling the crude sepia with a solution of soda, 
which dissolves the colour but not the bag ; the liquor is then 
filtered, and to the nitrate acid is added to precipitate the pig- 
ment; the precipitate is collected, washed, and dried. 

Sepia is a blackish-brown pigment of very line texture, mixing 
well with both oil and water. It is somewhat transparent, but 
its colouring power is very great, and it is capable of being so 



CAPPAGH BROWN. 229 

used as a water-colour as to show a great variety of tints and 
shades ; it is this property which makes it of value for mono- 
chrome work to artists; no other pigment is capable of being 
used in this manner with so much facility. 

Sepia is a compound of calcium and magnesium carbonates, with 
an organic colouring principle ; Prout has analysed it, and gives 
its composition as 

Melanin or black pigment, . . 78 '00 per cent. 

Calcium carbonate, Ca C O 3 , . . 10 "40 

Magnesium carbonate, Mg C 3 , . . 7 '00 ,, 

Alkaline sulphates and chlorides, . 2*16 ,, 

Organic mucus, . . . . . 0*84 ,, 

98-40 

The black pigment may be isolated by boiling the bag, first 
in water, which takes out the alkaline salts ; then in hydro- 
chloric acid, which takes out the calcium and magnesium car- 
bonates ; then, after washing with water, filtering, and drying, 
the pigment is ready for use. 

Sepia is a fairly permanent pigment, being but little affected 
by exposure to light and air. It is not altered by admixture 
with any other pigment. 

CAPPAGH BROWN. 

This pigment is of a reddish-brown hue, and much resembles 
raw umber both in appearance and composition. It is found at 
the Cappagh Mines of Lord Audley, which are situated about 
10 miles from the town of Skibbereen, in the county of Cork. 

Prof. A. H. Church gives * the following analysis of Cappagh 
brown : 



Water, given off at 100 C., 
Water, given off at a red heat, 
Ferric oxide, Fe. 2 Oa, 
Manganese dioxide, Mn 2 , . 
Alumina, A1 2 O& . 
Lime, Ca O, ... 
Magnesia, Mg 0, . . . 
Silica, Si 2 , ... 




18-7 per 
11-6 
34-4 
27-2 
2-6 
1-1 
trace. 
4-6 
0'4 


cent. 


Phosphoric acid, P 2 0$, 



There were traces of organic matter, but not enough to show 
that it had been derived from bog earth or peaty matter. It is 

* Church, Chemistry of Paints and Painting, p. 206. 



230 BROWN PIGMENTS. 

possible that part at least of the manganese, if not the whole of 
it, existed in the pigment in the form of the red oxide, Mn 3 O 4 . 

When heated above 100 C. it acquires a rich red colour not 
unlike that of burnt sienna. 

Cappagh brown works well in oil- or water-colour, and is a 
permanent pigment. Its qualities as an oil colour are mucb 
improved by a preliminary drying at a temperature not exceed- 
ing 75 to 80 C. It has been much used by artists since its 
introduction. 

MANGANESE BROWN. This brown is an oxide of man- 
ganese. It is prepared artificially from the waste still-liquors 
of the chlorine manufacturer, by precipitating these with sodium 
carbonate, collecting the precipitate, and calcining in a furnace 
to a low red heat, until samples taken out and allowed to cool 
show that the pigment has acquired the desired shade. It is a 
good and permanent pigment, but it has such excessively-strong 
drying properties as to make it very unusable as a pigment; 
hence it has gone out of use. The manufacture of manganese- 
brown was patented in 1871 by Rowan. 

COLOGNE EARTH, CASSEL EARTH, RUBENS 
BROWN, and other browns are pigments of natural origin 
of varied composition; some partake of the character of brown 
lignite of a soft character, others more nearly resemble the 
umbers in their composition. Their value as pigments is very 
variable, and, as their composition cannot be depended upon as 
being constant, it is advisable to avoid the use of these pigments 
for all artistic painting. 

BONE-BROWN is made by gently calcining bones until they 
acquire a brown colour ; it resembles bone-black in composition, 
but contains some undecomposed animal matter ; it is not much 
used. 

PRUSSIAN BROWN. This pigment is rarely met with 
now. It was prepared by gently calcining Prussian blue, and 
hence is a mixture of ferric oxide and carbon ; necessarily the 
pigment was a costly one, and it has no advantage over such 
pigments as umber or Vandyke-brown. 

BISTRE. This pigment is prepared from the soot of wood, 
especially from that of beech wood, which gives the finest quality. 
The soot is collected and washed with hot water until the latter 
does not extract any more soluble matter from it; sometimes 
the soot is subjected to a preliminary grinding before the 
washing. The bistre is dried, and is then ready for use. 
Bistre is not used as an oil-colour. It has a fine warm brown 
colour of a yellowish hue. Its permanence depends very much 



ASPHALT OR BITUMEN OF JUDEA. 231 

on the character of the wood-soot from which it is made ; some- 
times this contains much tarry matter which is not completely- 
extracted from the bistre by the washing operation ; the larger 
the quantity present in the bistre the more fugitive is the 
pigment. The tarry matter oxidises on exposure to light and 
air, and the tint becomes, in consequence, paler. 

ULMIN BROWNS are pigments made by heating organic 
matter with alkalies; they are not used, as they are too fugitive. 

ASPHALT or BITUMEN OP JUDEA was used as a pig- 
ment by many of the older artists; but, as time has brought out 
its many defects, artists have ceased to use it. It enters very 
largely into the manufacture of varnishes partly as a colouring 
matter, partly as a resinous matter. It is described in the 
section on varnish materials. 




OF THE 
IVERSITT 
; ALJFORNlA^ 



232 



CHAPTER VIII. 
BLACK PIGMENTS. 

NEARLY all the black pigments in use, certainly all those which 
are most used, are composed either of carbon itself or have that 
element as their colouring principle. Although carbon exists 
naturally, yet its native form is not used as a pigment in painting, 
as it lacks the properties required for that purpose ; therefore 
all the black carbon pigments are made artificially. 

Such carbon-blacks are known under a variety of names; lamp- 
black, vegetable black, carbon-black, are almost, especially the 
last two, pure carbon ; animal-black, bone-black, ivory-black, 
drop-black, Frankfort black, are blacks prepared from animal 
and vegetable matters, and contain various other constituents be- 
sides carbon. 

Besides the pigments just named, and which are specially 
prepared for use as pigments, carbon is also obtained in other 
forms, such as coke, charcoal, soot, &c. Some of these are more 
or less black, and they have been proposed for use as pigments 
after being subjected to grinding and washing, but they do not 
make good pigments, and it is doubtful whether they are so used 
at the present time. Certain natural minerals, such as coal and 
carbonaceous shale, have also been proposed to be used as pig- 
ments, but, as with those just noted, the proposal has probably 
never been put into practical use. 

Carbon is an elementary body belonging to the group of non- 
metals ; its chemical symbol is C, and its atomic weight 12. It 
is a combustible body in all its forms ; in burning it combines 
with oxygen to form carbon dioxide (carbonic acid gas), C O 2 , 
whence it follows that all the black pigments of which carbon 
forms the principal or only constituent are combustible ; a fact 
which sometimes makes itself apparent in a disagreeable form 
during the process of manufacture. It is a perfectly stable 
element, and will remain unaltered by exposure to the atmosphere 
for any length of time. 

Acids and alkalies have no action on carbon. All forms of 



LAMP AND VEGETABLE BLACKS. 233 

carbon-blacks are perfectly permanent pigments, and are unsur- 
passed in permanency by any other pigment. They can be mixed 
with all other pigments without bringing about any alteration. 

Some of the blacks have slight peculiarities, as will be noticed 
in the descriptions of them. 

LAMP AND VEGETABLE BLACKS. 

These two pigments are closely allied as regards the method of 
their preparation and their composition ; as a matter of fact they 
are made together at the same time and by the same operation. 
Lamp-black is probably the most common and most used of the 
black pigments. Essentially it is a kind of soot. Whenever a 
combustible body, such as an oil, or fat, or grease, is burnt under 
such conditions as to preclude complete combustion, then a large 
volume of smoke is produced, and this deposits a black soot on 
any surface it may come in contact with ; such soot has a strong 
black colour and is highly prized as a pigment. Owing to the 
fact that the earliest convenient means of producing this black 
was by burning the oil in a lamp under conditions, easily 
attained, which would ensure that the combustible would not be 
completely burnt, the black has derived its name of lamp-black. 
Ver}'- little lamp-black is now made by burning oil in a lamp, 
partly because materials are now used in its preparation which 
cannot be burnt with good results in a lamp. 

The materials used in the manufacture of lamp and vegetable 
blacks are very varied, and comprise all kinds of oils, fats, coal- 
tar oils, and greases ; in fact, anything that will yield a great 
deal of black smoke while burning, preference being given to 
those which are cheapest and least available for any other 
purpose. There are some differences in the quality of the blacks 
yielded by the different kinds of materials used ; the fatty oils 
and greases yield the best blacks; the hue is better and the 
black is finer and less greasy than that from any other kind of 
grease. The greases from coal-tar give fair blacks ; they are 
rather browner in hue than the blacks from the fatty oils, and 
more inclined to be oily from some of the material volatilising at 
the high temperature at which it is burnt. The residues from 
the distillation of shale give fair blacks, but are liable to contain 
traces of volatile unburnt matter. This oily volatile matter in 
the blacks from coal-tar and shale greases has the effect of causing 
the black to be a bad drier when used as an oil-paint. 

The process of manufacture of lamp-blacks consists essentially 
in burning the material and collecting the soot. The plant 



234 



BLACK PIGMENTS. 



required consists, then, of two parts 1st, a lamp or furnace in 
which the material is burnt ; 2nd, chambers in which the black 
collects. 

1st Method. An old method (not much used now, as it is 
only capable of making lamp-black from liquid oils which are 
comparatively costly) consisted essentially in burning oil in 
lamps and collecting the soot. One of the most modern forms of 
the plant used is shown in Fig. 24, from which it will be seen to 
consist of a lamp, A, constructed on the bird-fountain principle ; 
the shape of the lamp varies in different places, but the essential 
features consist of a wick-holder, a, constructed to burn a short, 
wide, or thick wick, to which a liberal amount of oil is supplied 




Fig. 24. Plant for making lamp-black. 



by the pipe, b, communicating with the bottom of the oil- 
container, cj by thus giving a plentiful supply of oil a very 
smoky flame results, which is the condition necessary to produce 
the largest amount of lamp-black. There should be a cup, d, at 
the bottom of the burner to catch any oil which may overflow 
from the top of the wick-tube. The oil-container, c, is so con- 
structed that it is quite air-tight when placed in position, and 
can only be supplied with air by an air-tube which connects the 
wick-tube with the oil-container ; this air-tube is small, and is so 
arranged that when the wick-tube is full of oil the opening into 
the tube is closed, thereby excluding the air, and so stopping the 
flow of oil from the oil- container, c; when, owing to the con- 
sumption of oil during the burning, the level of the oil in the 
wick-tube falls below the opening of the air-tube, then air passes 
into the oil-container and causes some oil to flow out into the 
wick-tube through the tube or pipe, b. By this means the supply 
of oil to the wick can be kept very uniform during the process of 



LAMP AND VEGETABLE BLACKS. 235 

manufacture, which uniformity is a necessary condition in the 
successful making of lamp-black. The collecting chambers consist 
of a series of strong cylindrical jute or linen bags, B, B, B, B, 
which alternately communicate with one another at the top and 
bottom, as shown in the drawing ; these bags are suspended by 
means of chains from a hook in the ceiling of the shed or room 
in which the operation is carried on. Over the burner of the 
lamp is placed a large funnel, C ; this opens into a large pipe, D ; 
from this proceeds a pipe, p, passing into the top of the first bag ; 
the soot from the lamp passes up the funnel and into the large 
pipe, D here some of the unburnt oily matter (which nearly 
always accompanies the soot) collects ; then the soot passes on 
into the bags, the heaviest black collecting in the first bags, while 
the finer black passes on into the last bags ; the heavier portions 
are sold as lamp-black, while the finer portions are sold as vege- 
table black. The bottoms of the bags are made to open, so that 
the black can be shaken from the bags into barrels placed under- 
neath. The bags may be made to communicate with one another 
by means of large curved tubes; or they may communicate by 
means of short, straight tubes placed near the top of the bags (see 
Fig. 24). The collecting bags are about 12 to 15 feet in length and 
about 3 feet in diameter. The last bag communicates with a 
chimney, so as to secure the necessary draught and ensure the 
black being drawn through the bags. It is usual to place a flue 
between the bag and the chimney; in this flue are placed a 
number of gauze frames on which collect the last portions of the 
black. Usually a number of these apparatuses are placed side 
by side. 

The black in the first two bags is kept separate from the black 
in the other bags, as it is liable to contain unburnt oil, which 
is a frequent cause of the black entering into spontaneous com- 
bustion. To prevent this occurring it is customary to calcine 
this oily black in a closed furnace, thereby destroying this oil and 
making the lamp-black better for use as a pigment. 

One of the objections to this form of plant is the inflammable 
character of the material with which it is made ; and as it is 
liable to take fire its use is becoming obsolete, especially as a 
great deal of waste greases from coal-tar, shale-oils, <kc., are used 
in the preparation of these blacks, which greases are not suitable 
for burning in lamps. 

2nd Method. The method most commonly used for the pro- 
duction of lamp and vegetable blacks, especially where heavy 
oils (such as creosote or anthracene oils from coal-tar and the 
residues of shale-oil distilling) are used, which will not burn 



236 BLACK PIGMENTS. 

in a lamp, is as follows : A long brick chamber (of varying size 
at different works, but averaging about 50 to 60 feet long by 
10 feet wide and 7 feet high) is constructed, having the roof flat 
or slightly inclined to one side. At one end is constructed a 
furnace similar to a common household boiler, with the fireplace 
outside the building ; the iron pan used is shallow, and, there- 
fore, exposes a great surface in comparison with the quantity of 
oily material used. A pipe from the exterior of the chamber 
opens just above the pan, and this is in communication with a 
reservoir of the oil. The interior of the chamber is divided into 
a number of compartments by brick partitions, which, however, 
do not extend quite across the chamber, but stretch alternately 
first to one side and then to the other of the chambers, thus 
compelling the smoke from the burning oil to take a circuitous 
course through the chamber before it passes out at the other 
end into the chimney of the works. 

The distance between each partition is about 4 feet, so that in 
a chamber 50 feet long there will be about twelve partitions ; the 
more there are of them the better will be the condensation of the 
lamp-black ; still, they must not be too narrow, or the workmen 
will not be able to get into them to remove the black. 

The process of manufacture consists in placing a quantity of 
oil, tfec., in the pan of the boiler, and heating it by means of a fire; 
when it gets hot enough it ignites when a light is applied, and, 
owing to the large extent of surface exposed, and, therefore, the 
limited amount of air in contact with it, it burns with a very 
smoky flame; as fast as the oil burns away the supply is kept up 
by allowing oil to drop through the pipe from the reservoir, 
care being taken not to supply the oil faster than it burns off 
in the pan, as this might cause the hot oil to overflow, and so 
produce a fire. The smoke and gases produced by the burning 
of the oil pass through the chamber, and the black gradually 
collects in the various compartments ; the coarser particles in 
the first compartments, and on the floor of the others; the finer 
particles on the roof of the middle compartments, and over the 
whole of the end compartments. Usually three qualities are 
collected heavy, medium, and light; the two former are sold 
as lamp-black, while the latter is sold as vegetable black. This 
black weighs about 6 Ibs. to a 40-gallon barrel ; that from the 
sides of the flues or compartments weighs about 8 Ibs., and the 
heaviest about 9 Ibs. 

From time to time the flues and compartments are swept out, 
the workmen passing from end to end of the compartments for 
that purpose; the bottom is swept first, then the sides, and 



LAMP AND VEGETABLE BLACKS. 



237 



finally the roof. The portion collected in the first compartment 
is frequently contaminated with uiiburnt oil, which has become 
volatilised by the heat of the furnace, and condensed again 
when it reaches a cold place. This oily lamp-black should 
always be collected separately, as the oil it contains often 
causes spontaneous combustion in the lamp-black. 

A different method of constructing the chamber is used in 
some works. In this case the partitions do not run from side 
to side, but from bottom to top, large openings being left alter- 
nately in the top of one partition and the bottom of the next. 
The chamber is of about the same size as the last described, 
and has a similar arrangement for burning the oil. This form 
of plant (shown in Fig. 25) necessitates a door being provided 
for every compartment, whereas in the last form of plant one or, 
at the most two, doors only are required. In some cases a large 
room is provided at the end of the chamber into which the 
gases, &c., pass before reaching the chimney ; here the last 
particles of black condense, so that gaseous matter only passes 
into the chimney. 




Fig. 25. Lamp-black chambers. 

The amount of lamp-black obtained by any of the processes 
just described varies very much; it depends upon the kind of 
oil or grease used, as well as on the completeness of the com- 
bustion and of the condensation, the effect of which on the yield 
must be obvious. Approximately the yield may be taken as 
25 to 30 per cent, of the weight of the oil used. The relative 
proportions of lamp and vegetable blacks produced vary con- 
siderably. 

3rd Method. The arrangements described above are those 
in general use, but are capable of very great improvement. A 
lamp-black furnace of an improved construction was patented in 
1879 by Messrs. Winslow, Humphrey & Buttrick. It is shown 
in Fig. 26. A number of these furnaces are usually arranged 



238 



BLACK PIGMENTS. 



in a row in front of condensing chambers, such as are used in 
the older methods. From the front wall, CT, of the condensing 
chamber projects an iron plate, f, which is supported on suitable 
brickwork; this forms the floor of the fire-chamber, e\ the pot, 
o, is kept at a red heat, and receives the oil or other suitable 




Fig. 26. Lamp-black furnace. 

material in drops from the pipe, I k, connected with the main 
supply pipe, m; this oil burns with a more or less smoky flame, 
and the products pass through the opening, h, into the condens- 
ing chamber. A quantity of the oil will drop on to the iron 
plate and enter into combustion; this serves to keep the plate 
and the pot, o, red hot. To prevent the temperature getting 
too high, a considerable air-space is left under the plate, while 
the openings, g g, admit, and allow the circulation of, air. i is a 
door by means of which the admission of air for the combustion 
of the oil is regulated. A comparatively low temperature is 
required and should be just above the burning point of the oil; 
this, however, varies, some oils burning below 500 F. and others 
above 600 F. 

The whole arrangement being outside the condensing chamber 
is readily accessible for the purpose of regulating the amount of 
oil, the temperature of the furnace, and other details affecting 
the production of the black. 

4th Method. A somewhat different method was patented 
in 1880 by Nawrocki. The oily matters used were burnt in a 
series of lamps arranged in a row, the oils being heated, if 
necessary, to ensure proper combustion. Above the burners 
was arranged an iron plate, against the bottom of which the 



LAMP AND VEGETABLE BLACKS. 239 

flames from the burners impinged; consequently, the combustion 
was rendered incomplete. This plate is circular in form and 
made to revolve; it is kept as cool as possible by the upper 
surface being divided by ribs into a number of compartments 
through which a constant stream of cold water is made to now. 
The action of the apparatus is as follows : The lamps are lighted 
and the plate caused to revolve; the black forms on the plate 
where it is in contact with the oil flames, while the revolution 
of the plate continually brings a fresh surface for the deposition 
of the black; as the plate revolves it comes into contact with 
scrapers, which scrape off the black into suitable receptacles, so 
that a fresh, clean, and cold surface of the iron plate is always 
exposed to the flames, and the maximum amount of black is 
produced by the combustion of the oil. 

The process is a slow one, and the yield of oil-black is not so 
great as in the processes above described. The black obtained 
in this way is rather more granular in its formation, and has a 
tendency to be somewhat harder and greyer in tint. Should, by 
any accident, any of the black be exposed to the action of the 
flames for too long a period, it is liable to be overburnt, and is 
thereby rendered hard and almost unusable as a pigment. 

5th Method. In America a large quantity of black is pro- 
duced by the combustion of the natural gas which flows out of 
the ground in many of the oil regions. This black is made in 
large quantities, and sold under the names of gas-black or 
carbon-black. The principle on which its manufacture is based 
is the same as that of the last method viz., the cooling of the 
flames of the burning gas by iron plates. The form of these 
plates has altered during the time which has elapsed since the 
industry was introduced ; at first stationary plates, with long 
trough-shaped upper surfaces, were adopted, and were kept cool 
by means of a current of water ; these were found to be subject 
to certain defects. Thus, water was condensed on the lower 
surface, which interfered with the proper condensation of the 
black, and made the latter damp ; then, the black formed, not 
being' immediately removed from the action of the flames, was 
burnt, and became granular in form, greyer in tint, and much 
harder. For the purpose of removing the black from the plates, 
scrapers were passed over the depositing surface from time to 
time. Of late, these fixed depositing plates have been replaced 
by revolving plates or cylinders which, during the time the 
black is being deposited, automatically revolve, and so the black 
as it is formed is removed from the action of the flames, and, 
therefore, cannot be overburnt. As the plates or cylinders 



240 



BLACK PIGMENTS. 



revolve, they come into contact with fixed scrapers, which 
remove the black from the surface as fast as it is formed. 
Carbon-black differs from oil-lamp blacks in being granular in 
form, and rather denser; it is blacker in hue than any of 
the other black pigments. For grinding this black, steel mills 
are preferable to stone mills. It is the purest form of carbon- 
black, made quite free from any trace of the unburnt oil often 
present in lamp-blacks, and from any trace of mineral matter. 
(See Oil and Colourman's Journal, June, 1891.) 

PROPERTIES AND COMPOSITION OF LAMP- 
BLACKS. Lamp-black is in the form of a black, flocculent 
powder with a fine texture ; in hue it is, usually, what is termed 
a jet-black, although some samples have a faint brownish tinge. 
It has great colouring and covering powers. It is rather difficult 
to mix with various vehicles, especially with water, but, when 
mixed, it works well as a paint ; it dries rather badly when used 
as an oil paint, especially those samples which contain unburnt 
oil. It is perfectly permanent as a pigment. It is sold in the 
form of powder and also as a paste ground with oil, of which it 
takes 27 per cent. ; or with turps, of which it takes 55 per 
cent, to form a stiff paste. 

Vegetable black is a more voluminous black than lamp-black, 
of a deep jet-black colour and very fine texture. Its colouring 
and covering powers are rather greater than those of lamp-black. 
Carbon-black has similar properties to vegetable black, but has 
a slightly more granular structure. 

These blacks are used for making printing inks, varnishes, 
paint, and for other purposes where a fine black pigment is 
required. 

Lamp-black consists almost entirely of carbon, but there is a 
small quantity of moisture and mineral matter present in all 
samples. Vegetable and carbon blacks are nearly pure carbon. 
The following analyses of lamp and vegetable blacks will show 
their average composition : 





Lamp-Blacks. 


Vegetable Blacks. 


. 1. 


2. 


1. 


2. 


Carbon, 

Water, - 
Ash, - 


93-5 
4-4 
2-1 


94-994 
1-806 
3-200 
_* 


98-87 
0-71 
0-42 


99-7 
0-3 



BONE-BLACK. 241 

Samples of lamp-blacks are sometimes met with containing 
small quantities of oil ; these come from the first condensers ; 
such lamp-black is defective, for two reasons 1st, the oil pre- 
vents the paint made from the black drying properly ; 2nd, the 
oil is liable to cause spontaneous combustion, so that it is no 
uncommon occurrence to find a cask of lamp-black almost red 
hot after standing some time. 

ASSAY AND ANALYSIS OP LAMP-BLACKS. The 
blacks just dealt with can be assayed for colour, colouring power, 
and covering power by the usual methods. Lamp-blacks should 
contain but a small quantity of mineral matter, not exceeding 
about 3-5 to 4 per cent. To determine the amount, 2 grms. should 
be weighed into a platinum crucible, and the black heated over a 
Bunsen burner until all the carbon is burnt off, and nothing but 
a greyish ash remains ; the crucible and its contents are now 
weighed, and the weight of the ash ascertained. Water can be 
determined in the usual way. The difference between the 
amount of water and ash and 100 may be taken as carbon. If 
the black shows signs of its containing oil, the amount of this 
impurity may be ascertained by treating a known weight, say 
about 5 grms., with petroleum ether in a Soxhlett extraction 
apparatus. The petroleum ether will take out the oil; the 
ethereal solution is run into a weighed glass, and the ether 
evaporated off in the water bath ; the combined weight of the 
glass and oil is taken, arid the amount of oil ascertained. Vege- 
table blacks are analysed in the same way ; they should have no 
oil, and should not contain more than 0'5 per cent, of water, or 
0'25 per cent, of ash. Carbon-blacks should resemble vegetable 
blacks in their composition. 

BONE-BLACK. 

Bone-black, or animal black as it is often called, is prepared 
from bones by a process of charring them in a closed vessel ; the 
organic matter they contain is decomposed, much volatile matter 
is given off, and carbon is left behind along with the mineral 
matter of the bones. 

The process of making bone-black may be carried on under 
two conditions 1st, the volatile products are not collected; 
2nd, they are collected. The first plan is the one generally 
adopted, as there is very little use for the oily matter, known as 
Dippel's or bone oil, which is obtained by the dry distillation of 
bones. 

1st Process. When the volatile products from the charring 

16 



242 BLACK PIGMENTS. 

of the bones are not collected, the black is obtained by breaking 
up the bones into small fragments and placing these in clay 
crucibles, fitted with a cover, which is not fastened down ; this 
arrangement allows the volatile matter to escape, but excludes 
the air, the access of which would cause the carbon to burn 
away. Any kind of furnace for heating crucibles will answer. 
One form of such furnace is shown in Fig. 27. The furnace has 
a flat hearth, measuring 7J yards long by 5 yards wide, and in the 




Fig. 27. Bone-black furnaces. 

centre is placed the fireplace, which is fed from the outside. A 
low firebrick arch extends over the hearth, from the bottom of 
which a number of flues pass round to the top of the arch, where 
they are in connection with the single large flue that carries 
away the products of combustion of the fuel used and the small 
proportion of volatile matter from the bones which is not burnt 
up in the furnace. Doors are provided in the sides of the furnace 
and the arch for the purpose of introducing the crucibles. The 
crucibles are made of fireclay, and are usually eighteen in number; 
each is provided with a lid ; in some places this is simply placed 
loose on the crucible, in others it is luted on with clay, a few 
holes being left for the purpose of permitting the escape of the 
volatile matter from the bones. When all the crucibles are 
placed in position in the furnace the doors are bricked up and 



BONE-BLACK. 243 

the fire lighted. The temperature is slowly raised to a red heat, 
at which it is maintained for from six to eight hours. The 
temperature should be kept as uniform as possible during the 
whole of this period ; at the end the fire is withdrawn and the 
furnace allowed to cool down. Comparatively little fuel is 
required as the bones give oiF a good deal of combustible matter, 
which assists in heating the furnace. 

When the furnace is sufficiently cool, the doors are opened and 
the crucibles withdrawn ; it is not usual to allow the furnace to 
cool down completely, but only sufficiently so as will allow the 
workmen to enter and work comfortably ; to let the furnace get 
quite cold means a loss of time, and, therefore, fewer charges 
worked in a given time, as also a greater consumption of fuel to 
heat up the furnace to the required temperature. It is important, 
however, that the crucibles should not be opened until they and 
their contents are quite cold ; if opened while they are hot, 
the black contained in them would take fire and burn away, 
thus leading to a loss of material. When the furnace is emptied 
of one charge it is ready for filling with a second charge. Two 
charges can be easily worked in a day in such a furnace, and, by 
working hard, three charges may be got through, each charge 
being about half a ton, and yielding about 60 per cent, of its 
weight of black. 

After the black is taken from the crucibles it is ground in a 
mill, care being taken to exclude grit and other materials liable 
to generate sparks by friction and thereby to set fire to the black. 
If the black is to be used as a pigment, it is ground into a fine 
powder ; if it is to be used for decolorising purposes, it is ground 
into a coarse granular material. 

2nd Process. When it is desired to collect the volatile 
bodies given off during the process of charring the bones, the 
operation is usually carried out in earthenware retorts, such 
as is used in the distillation of coal or wood; a red heat being 
used. The volatile portions are passed through a series of 
condensers formed of iron tubes exposing a large surface to 
the cooling action of the air, by which operation they are 
separated into three portions, viz. : (1) an aqueous liquor 
containing a fair proportion of ammonia, which can be recovered 
by the usual methods ; (2) oily or fatty matter having a very 
peculiar and unpleasant odour, which may be used for preparing 
lamp-black, and is used, to a small extent, for currying leather ; 
and (3) an uncondensable gas, which can either be discharged 
into the atmosphere or used, if necessary or thought desirable, 
as fuel for heating the furnaces. 



244 BLACK PIGMENTS. 

This method of preparing bone-black is rarely used. It is 
important that after the operation is ended that the fires be 
withdrawn, and the retorts be allowed to cool down completely 
before they are emptied ; if opened while hot the black will take 
fire. 

PROPERTIES AND COMPOSITION OF BONE- 
BLACKS. Bone-black, as sent out for use as a pigment, is a 
fine powder, of a greyish-black hue, varying a great deal in 
various samples. It has not the brilliant hue of lamp-black, 
nor its depth of colour. As a pigment it is quite permanent, 
and works well in both oil and water, mixing easily with both 
these vehicles. It is a slow drier when used as an oil-paint. 
The chief use to which bone-black is put is in the preparation 
of blacking, where the large quantity of calcium phosphate and 
carbonate it contains gives to it properties which are as impor- 
tant, perhaps even more important, than its colouring powers. 

While bone-black owes its colour to carbon, yet it consists 
principally of calcium phosphate and carbonate derived from 
the mineral constituents of the bones. The following analyses, 
made by the author, of bone-blacks will give some idea of their 
mineral composition : 

1. 2. 3. 

Welter, 24-12 6'19 7'11 

Ash or mineral matter, . . 44'18 77 "97 77'75 
Carbon, 31 "7 15 '84 15 '14 

The first sample contained an abnormal quantity of water for 
a black pigment, and also contained some lamp-black. 

The average quantity of ash contained in bone-blacks is 75 
per cent., of which 60 per cent, is phosphate of lime, the rest 
being carbonate of lime, with traces of silica, iron, and alumina. 
The carbon ranges from 15 to 20 per cent. As a rule, the ash 
is white, or of a pale greyish tint; sometimes it has a red 
tint showing that it contains iron. The author has some 
suspicion that blacks leaving a reddish ash are not pure bone- 
blacks. 

ASSAY AND ANALYSIS OP BONE-BLACKS. Bone- 
blacks can be assayed for tint or colour, colouring power, &c., by 
the usual methods. An analysis of it may be made in the same 
way as described under lamp-black. The mineral matter may be 
further tested if there are suspicions that the sample under 
analysis has been made by mixing lamp-black with some mineral 
matter. The ash which is left behind on heating the black in a 
platinum crucible should almost entirely dissolve in strong hydro- 
chloric acid without much effervescence ; on adding ammonia a 



ANIMAL BLACK. 245 

copious white precipitate should be obtained ; on drying and 
weighing, this should amount to about 60 per cent, of the weight 
of the black. To the nitrate from this precipitate ammonium 
oxalate should be added and the resulting precipitate filtered off, 
dried, and weighed. The amount of it should be about 15 per 
cent, of the weight of the black. The precipitate with ammonia 
should be powdery, not flocculent, in appearance ; the latter 
would indicate the presence of alumina, of which traces only are 
present in the ash of pure bone-blacks. 

IVORY-BLACK. 

Ivory-black is, or should be, made from the waste cuttings of 
ivory by the same process as bone-black is made from bones. 
Much bone-black is sold as ivory-black. In composition and 
properties ivory-black resembles bone-blacks. The following are 
some analyses, made by the author, of ivory-blacks, but the 
genuineness of the samples is not guaranteed : 

1. 2. 3. 

Water, .... 7'05 7'41 8'27 

Mineral matter, . . 75'58 76'21 74'86 

Carbon, . . . 17 '37 16 '38 16 "87 

Ivory-black has usually a finer and more brilliant hue than 
bone-black. It is used for making printing ink, blacking, <kc. 



ANIMAL BLACK. 

Under the names of animal black and animal charcoal a num- 
ber of black pigments, made from animal matters of all kinds and 
by various processes, are sold for use as pigments and for de- 
colorising purposes. Animal black is made, much in the same 
manner as bone-black, from all kinds of animal products, waste 
pieces of leather, skins, hoofs, horns, hair, <fec. In the process of 
making yellow prussiate of potash there is produced a large 
quantity of black, which was at one time thrown away, but it is 
now largely used for decolorising sugar, paraffin wax, &c. 

The composition of animal black is very variable ; the following 
are analyses of several samples of these blacks sent to the author 
for analysis : 

1. 2. 3. 4. 5. 6. 7. 

Water, . 2875 28*11 3670 45'50 24*12 2775 6'19 

Ash, . 29-22 3071 34 -80 24 "70 44-18 074 77 '97 

Carbon, . 42-03 41-18 28'50 29'80 3170 71'51 15'84 



246 BLACK PIGMENTS. 

No. 6 had a most peculiar fetid odour and contained a quantity 
of unburnt oil ; from its appearance and composition it is evident 
that its only claim to the name of animal black was that it was 
made by the lamp-black process from bone oil. 

No. 7 was a true bone-black. With these two exceptions the 
above analyses show that this black is made from a variety of 
materials. 

Animal black can be used for all purposes to which black 
pigments are applied. 



FRANKFORT OR DROP-BLACK. 

This black is named " Frankfort black," because it was first 
prepared in the old German town ; " drop-black," on account of 
the shape into which it is made up for sale. 

Drop-black is made from a great variety of materials of an 
organic character, such as vine twigs, refuse of wine-making, 
peach stones, hop bine, bone shavings, ivory cuttings, &c. These 
are calcined in a closed vessel until they are thoroughly charred. 
The black so obtained is then ground up as fine as possible with 
a little water ; then the mass is lixiviated to free it from soluble 
matters, and dried. Then it is mixed with a little glue water 
and made up into pear-shaped drops for sale, for which purpose 
they are ready when dry. 

Drop-black is a black of fine texture, varying in hue from a 
bluish-black to a somewhat reddish-black, which is due to the 
different materials of which it is made ; vegetable matters yield 
a black of a bluish hue, while animal matters give a black of a 
greyish hue. 

Drop-black owes its colour to carbon, the amount of which 
varies in different samples ; it also contains some mineral matter 
which will vary in amount and kind according to the character 
of the material from which the black was prepared. The 
following is an analysis by the author of a sample of drop-black: 

Water, 2 '333 per cent. 

Carbon, . . . . 65'742 

Mineral matter, . . . 31 '925 

The mineral matter contained phosphate of lime, which showed 
that bones had been used in making this sample. 

Drop-black is used for all purposes for which black pigments 
are required. 

German black is a synonym for drop-black. 



MISCELLANEOUS BLACKS. 247 



MISCELLANEOUS BLACKS. 

Besides the black pigments described above, several other 
substances may be briefly noticed, which have been proposed 
for use as pigments, or have been so used on a small scale. 

CANDLE BLACK is a kind of lamp-black made on a small 
and extremely limited scale from the flame of a candle by holding 
a cold plate over it. 

PRUSSIAN BLACK is made by calcining Prussian blue in 
a closed vessel until the residue has a black colour; this black 
is a mixture of carbon and oxide of iron, and usually has a 
brownish tinge; it possesses no advantage over lamp-black and 
is more expensive. 

BLACK LAKE. When a solution of sulphate of copper and 
bichromate of potash is added to a decoction of logwood a black 
precipitate falls down; this, after being washed and dried, forms 
black lake. Too much bichromate of potash should be avoided, 
as it has a tendency to turn the lake grey; the substitution of a 
little sulphate of iron for some of the bichromate improves the 
lake, while reducing the risk of adding too much of the chrome 
salt. Black lake is a pigment of a fine hue and texture; it is 
not permanent when used as a pigment, fading on exposure 
to light and air. 

TANNIN-BLACKS are similar pigments to the last, and 
are made by adding solutions of either the sulphate or the so- 
called " nitrate of iron " to solutions of tannin materials, such 
as sumach, divi-divi, myrobolans, &c. Their production from 
waste leather has been made the subject of a patent; the leather 
scraps are boiled in an alkaline solution, which dissolves out the 
tannin matter used in tanning the leather; to this solution a 
mixture of alum and sulphate of iron is added and the black 
precipitated. The tannin- blacks have a bluish hue; they have 
no great colouring power, and are not permanent when exposed 
to light and air. They have been very little used as pigments. 

CHARCOAL-BLACKS. These are made by grinding the 
charcoal obtained by charring soft, woods. The grinding should 
be well done, and the black pigments should be washed with 
water to remove any soluble matters it may contain. These 
blacks are mostly sold as lamp-blacks, carbon-blacks, etc. In 
their general properties they resemble the lamp-blacks, but are 
a little more granular in texture. 

COAL-BLACKS have been made by grinding coal and shale, 
but it is doubtful whether any are now used. 

LEAD-BLACK is the sulphide of lead made, according to the 



248 BLACK PIGMENTS. 

patent specification, by taking lead-fume (i e., the powder which 
collects in the flues of lead furnaces) and boiling it for some time 
with a solution of sodium sulphide. The black soon forms. It 
is doubtful whether this black has been used on a practical scale. 
Its permanency is doubtful, as it is possible that it may be prone 
to oxidation to lead sulphate and, therefore, to decolorisation. 

PRUSSIATE BLACK is the black residue (consisting mostly 
of carbon) which is obtained as a bye-product in the manufacture 
of yellow prussiate of potash. This is collected and well washed 
with water, when it is ready for use. It is largely used as a 
decolorising agent. 

MANGANESE-BLACK is the oxide of the metal manganese 
found naturally and simply ground up very fine for use as a 
pigment. The great objection to its use as a pigment is its great 
drying properties. It is also expensive, and possesses no material 
advantage as a pigment over lamp-black. 

Blacks have been proposed to be made from other materials 
by mixing ochres with peat and similar carbonaceous materials, 
and calcining the mass in closed vessels. Spanish black is a 
name given to a black made from cork shavings. The use of the 
black sulphide of iron was patented by Glaus in 1882, but it has 
never come into practical use. Aniline-black has been proposed 
to be used as a pigment, but its great expense is against its 
practical use for this purpose. It is made by dissolving aniline 
hydrochlorate in water, and adding to it a solution of potassium 
bichromate acidified with sulphuric acid. The black precipitate, 
which rapidly forms, is collected and washed. 



249 



CHAPTER IX. 
LAKES. 

LAKES form a class of pigments of considerable use in painting. 
They were among the pigments used by the early Italian 
painters, from whom their use has descended to the present 
time. Pliny gives some account of them, and from this descrip- 
tion the origin of the name " lake " can be gathered. The early 
Italian dyers for certain colours used what was known as " lac," 
which was either the product now known under this name, or an 
analogous body. This lac requires the aid of tin and alumina com- 
pounds before the colour can be developed and fixed on the fabric 
which has to be dyed ; during the process of dyeing some of the 
colouring matter of the lac combines with some of the tin and 
alumina to form an insoluble body, which forms a kind of 
coloured scum on the top of the dye- vat ; this substance, known 
to the Italian dyers as " lacca," was collected, dried, and sold 
to artists. In the same way other laccse were obtained when 
other natural dyestuffs were used ; gradually methods of pre- 
paring these laccse were discovered, by which they could be 
obtained direct from the dyestuffs themselves, without the 
necessity of troubling the dyer, and thus has arisen the prepara- 
tion of the lakes, which name can be readily traced to the laccse 
of the Italian dyers. 

Lakes may be denned to be compounds of an organic colouring 
principle with a metallic body. The organic colouring principle 
may be obtained (as it was in the early times, and until very 
recent years) from natural colouring matters, such as lac, cochi- 
neal, Persian berries, fustic, Brazil wood, sapan wood, &c. ; or it 
may be derived from the coal-tar colours, a source which has 
only lately come into prominence for lake-making, but which 
promises in the future to supplant the natural colouring matters 
for this purpose, as they very nearly have done for dyeing textile 
fabrics. 

The colouring principle of most natural colouring matters is of 
an acid or phenolic character, and will combine with bases, such 



or THE 
tJ N I V T. T3 o T r -or 



250 LAKES. 

as tin, alumina, iron, lead, antimony, &c., to form coloured bodies 
which are insoluble in water ; as a rule, the affinity between the 
two bodies is so great that the lake is precipitated when a solu- 
tion of a metallic salt is added to one of the colouring matter. 
Theoretically, a lake should be a compound of the colouring 
principle and the metallic base combined in equivalent pro- 
portions ; but, practically, such a lake does not exist ; usually, 
the base largely predominates. This excess is sometimes acci- 
dental, but often purposely made, the object being to modify the 
shade of the lake, as is the case with Dutch pink, rose pink, and 
one or two others. Then, again, in some lakes there may be 
small traces of the colouring principle carried down mechanically 
with the lake during the process of making. 

Lakes are usually made by preparing a decoction of the colour- 
ing matter, and then adding to this a solution of the base ; as a 
rule, the lake forms almost at once ; at other times, the addition 
of a small quantity of a solution of carbonate of soda is sufficient 
to throw down the lake. By preparing alkaline solutions of the 
colouring matter the lake is thrown down at once on adding the 
solution of base ; this method is not always applicable, as the 
alkali sometimes affects the shade of the resulting lake, as in 
making alizarine-lakes. 

The colouring matters, or rather their colouring principles, may 
be divided into two groups. One contains coloured matter, and 
includes such substances as fustic, Persian berries, and cochineal, 
which may be called substantive colours, as the colour does not 
depend upon the mordant or base used ; thus Persian berries 
will give a yellow lake with either alumina, tin or lead, although 
there are some minor differences in the tint or shade of the 
yellow so produced. The other group may be called adjective 
colours. It comprises substances like alizarine, fresh logwood, 
&c., in which the colour is only developed when the colouring 
principle is combined with a base, and differs with the base 
used ; thus alizarine, when combined with alumina gives a red, 
while with iron it gives a deep dull violet ; again, logwood with 
antimony gives a violet, with iron a blue-black, and with chrome 
a deep blue. 

All lakes should be quite insoluble in any vehicle, such as 
water, oil, turpentine, or spirit, used to make them into a paint ; 
on the other hand, a true lake is always more or less transparent 
when used as a pigment, and lakes are, therefore, mostly used as 
covering or glazing colours to modify the tint of an under cont 
of paint, and to obtain effects which are not obtainable with 
opaque pigments. Some lakes are rendered nearly opaque by 



CARMINE. 251 

mixing the materials during the process of manufacture with 
some opaque white pigment, by which the body or covering 
power of the lake is increased, and at the same time the shade 
is more or less affected. If a lake dissolves in the vehicle, then 
all its properties as a pigment in regard to its body or covering 
powers are lost, and a coloured varnish only is obtained which 
will not do the work it is intended that the lake-paint should 
do. 

In the following pages the lakes prepared from the natural 
colouring matters will first be described, then those prepared 
from the coal-tar colours. 

RED LAKES. These can be prepared from cochineal, 
madder, Brazil wood, bar wood, and one or two other natural 
products; but those named are what are chiefly used commer- 
cially. Cochineal yields carmine, crimson, scarlet, Florentine, 
and a few other lakes. Brazil wood gives rose-pink and some 
of the cheap red lakes. Madder yields the madder lakes so 
much prized by artists, but which are too expensive for common 
house-painting. The other red colouring matters are, owing to 
special difficulties, not much used in lake-making. It is not 
intended to give a special description of colouring matters used 
in the preparation of lakes; if the reader requires such, he 
should refer to some work on dyeing, such as that by Knecht, 
Rawson & Lowenthal. 

CARMINE. The best example of a lake is probably the 
pigment carmine, which is an almost pure lake, a combination 
of the colouring principle of cochineal (carminic acid) with 
alumina and tin. The exact method by which it is made from 
cochineal has never been published, and is probably only known 
to the few makers of this lake, although various descriptions of 
processes have appeared from time to time; some of these are 
very misleading, and have probably been intentionally made so. 
The pigment has bee a known for more than 200 years; its dis- 
coverer is unknown, although according to one statement it was 
a Franciscan monk. In 1656 a writer named Homberg pub- 
lished a recipe for making it. 

The chemical nature of carmine is even now but imperfectly 
understood, although it and its source cochineal have frequently 
been examined. In the main, the various writers agree that 
carmine is a combination of the colouring principle of cochineal 
(carminic acid) with alumina; but there is always present small 
quantities of other bodies, which renders any investigation into 
its composition a matter of some difficulty. 

Liebermann gave, in the Berichte der Deutsche Chemiscl 



252 



LAKES. 



Gesellschaft, vol. xviii., p. 
cochineal-carmine : 

Moisture, 

Nitrogenous matter, 

Ash, 

Colouring matter, . 

Wax, 



1,969, the following analysis of a 



The ash contained 



Stannic oxide, Sn Og, 
Alumina, A1 2 3 , . 
Calcium oxide, Ca 0, 
Magnesia, Mg O, . 
Sodium oxide, Na 2 0, 
Potassium oxide, K 2 0, 
Phosphoric acid, P 2 5 , 



17 per cent. 
20 
7 
56 
traces. 



0'67 per cent. 
43-09 
44-85 

1 02 

323 

3-56 

3-20 

99-62 



More recently, in the Journal fur Praktische Chemie, 1890, 
No. 3, Lafar published the following analysis of carmine : 



Water, 
Ash, 



As sold. 

15*50 per cent. 
6-87 



Nitrogenous matter, 23 '26 
Colouring matter, 54 '37 



The ash contained 



Copper oxide, Cu 0, . 
Stannic oxide, Sn O 2 , . 
Alumina, A1 2 3 , 
Ferric oxide, Fe 2 3 , 
Calcium oxide, Ca 0, . 
Magnesia, Mg 0, 
Sodium oxide, Na 2 0, . 
Potassium oxide, K 2 O, 
Phosphoric acid, P 2 65, 
Silica, Si 2 , 
Carbonic acid, C 2 , . 



Dried. 

... per cent. 
8-14 
27-60 
64-26 



0-35 per cent. 

014 
40-48 
traces. 
44-20 

0-61 

5-40 

3-20 

2-71 

0-60 

2-31 



The two analyses agree with one another as well as analyses 
of a commercial and variable product like carmine can be 
expected to agree. The lime and alumina in the ash are in 
the proportion of 2 CaO : A1 2 O 3 ; this circumstance would point 
to the fact that carmine is not purely an alumina lake, but a 
lime-alumina lake, with some proteid matter. The tin and 
copper in the ash have probably been derived from the vessels 
in which the cochineal has been boiled, as it is often recom- 
mended in recipes for carmine-making, to make the decoction 



CARMINE. 253 

of cochineal in two vessels; the other constituents are of no 
importance. 

The following methods have been published for preparing 
carmine : 

1. 1 Ib. of cochineal is extracted by boiling in water for from 
15 to 20 minutes, the decoction is strained off, 1 oz. of alum is 
added, and the boiling continued for a few minutes longer ; the 
clear liquor is decanted off and 1 oz. of cream of tartar added ; 
the mass is then allowed to stand for the carmine to settle. 

2. Boil up 2 Ibs. of cochineal, strain off the decoction, add 
2 ozs. of alum, 3 ozs. of muriate of tin (a solution of stannous 
chloride), 2 ozs. of carbonate of soda, and allow to stand for 

2 days, when the carmine will have been thrown down. 

3. 1 Ib. of cochineal is boiled with water and J oz. of carbonate 
of soda ; to the decoction is added 1 oz. of alum and 3 drams of 
cream of tartar; the mixture is allowed to stand for the carmine 
to be deposited. 

4. The following process was patented by Wood, in 1856 : 
9 ozs. of sodium carbonate, 8 ozs. of citric acid, and 27 quarts of 
water, are boiled together; then 1^ Ibs. of cochineal are added 
and the mixture boiled for 1| hours, strained, and clarified; the 
liquor is heated to the boil and 9 ozs. of alum are added ; the 
mass is then boiled for 5 minutes longer and allowed to stand for 

3 days, when the carmine precipitated is collected, washed, and 
dried. 

A recipe was published by Madame Cenette, of Amsterdam, a 
noted maker of carmine, but this is defective, and carmine cannot 
be made by following it. 

In the preparation of carmine it is advisable to use tin or 
tinned-copper vessels for boiling the cochineal in, as a small 
quantity of the metal dissolves in the liquor and exerts a 
beneficial influence on the carmine which is produced. Earthen- 
ware vessels may be used, but iron must be avoided, as any trace 
of iron in solution affects the shade of the carmine rather injuri- 
ously. The use of too much alum should be avoided, as it tends to 
reduce the colouring power of the carmine and also to alter the 
tint, turning it more crimson, while the shade ought to be a 
scarlet. 

Carmine is a deep fiery-scarlet powder, slightly varying in 
tint; the best quality is known commercially as " nacarat car- 
mine." It is insoluble in water, alcohol, ether, turpentine, and 
all the vehicles used in mixing paints, but soluble in strong 
mineral acids. In caustic soda, caustic potash, and ammonia 
solutions, it dissolves with a deep crimson colour, from which 



254 LAKES. 

solutions the carmine can be precipitated by exposure to the air 
or by the addition of weak acids like acetic or tartaric acid ; the 
carmine so obtained is very little changed from the original, so 
far as tint is concerned. Solutions of salts of iron, lead, Ac., 
have an injurious effect on the tint of the carmine. Carmine, on 
being heated in a crucible, burns and leaves behind from 7 to 
10 per cent, of a white ash, which consists principally of alumina 
and lime, as will be seen on examination of the analyses given 
above. 

As a pigment carmine works well in either water or oil, and 
is used, to a small extent, by artists as a glazing or tinting 
colour. It is not permanent, as a few months' exposure to sun 
and air is sufficient to impair the brilliancy of its hue, and pro- 
longed exposure causes it to fade. 

Carmine is frequently adulterated with other lakes and red 
pigments. The fact of adulteration may be ascertained by treating 
the lake with ammonia, when, if pure, it will completely dissolve ; 
if not pure, the adulterant is left as an insoluble residue. 

CARMINE LAKE. In preparing carmine the whole of the 
colouring matter of the cochineal is not precipitated, and therefore 
the liquors from the carmine are strongly coloured, and are 
utilized for the preparation of carmine lake. The usual method 
of making is to add to the liquors a small quantity of alum (about 
one-fourth of the weight of the cochineal used in making the 
liquor originally), a little tin chloride, and then sufficient potash 
carbonate solution to precipitate the whole of the alum and tin; 
the precipitate is collected, washed, and dried. 

Ajaother method is to make a decoction of cochineal, and to 
add to this freshly-precipitated alumina (obtained by adding 
ammonia to a solution of alum, and collecting the precipitate 
of alumina); the colouring matter is absorbed by the alumina, 
and a lake is formed; only sufficient of the alumina is added 
as will produce a lake of the required depth of colour. 

1 Ib. of cochineal is boiled in water, and 1 Ib. of cream 
of tartar or carbonate of potash is added; then 1 oz. of tin 
chloride is added, and sufficient alum to throw down all the 
potash salt and the colouring matter of the cochineal; the lake 
obtained is collected, washed, and dried. 

FLORENTINE LAKE is carmine lake which is, after making, 
mixed with a small quantity of gum water and moulded into the 
form of tears or drops. Sometimes cheap qualities of Florentine 
lake are prepared by mixing the genuine article and a lake made 
from Brazil wood together. 

Crimson-Lake and Scarlet-Lake are prepared from cochineal 



RED LAKE. 255 

in the same way as carmine-lake, but the proportions of alum 
and tin used differ, and the precipitated lake is moulded into 
the form of drops. 

BRAZIL -WOOD LAKES. 2 Ibs. of Brazil wood are 
digested in 8 gallons of water for 24 hours ; then the mass is 
boiled for half-an-hour or so, strained, and allowed to stand for 
a few days or a week ; this is necessary if the full strength of 
the wood is to be utilised; the colouring principle in fresh 
Brazil wood exists in the form of brazilin, which of itself has 
little colour; but by oxidation it is converted into brazilein, 
which possesses strong colouring power; this change occurs 
when decoctions of the wood are allowed to stand for some time. 
When the decoction is ready there is added to it 1 \ Ibs. of alum, 
| Ib. of tin chloride solution, and then sufficient carbonate of 
soda to precipitate. The precipitate is collected, washed, and 
dried. 

Another method is to prepare a decoction as before, and to 
add to this sufficient freshly-precipitated alumina and oxide of tin. 

A variety of Brazil-wood lake, which is known as Vienna lake, 
is prepared by mixing 60 Ibs. of starch, 20 Ibs. of chalk, and 2 
Ibs. of gypsum with a decoction of Brazil wood ; then 2 Ibs. of 
ground alum are added, and the mixture stirred, and allowed to 
stand for 12 hours; the liquor, which will be fairly free from 
colour, is decanted off, fresh alum and decoction of Brazil wood 
added, and the stirring, standing, &c., a repeated ; this operation 
is continued until the lake has acquired the desired shade. At 
first the tone is inclined to be violet-red, owing to the alkalinity 
due to the chalk ; but as this becomes neutralised by the succes- 
sive additions of alum, the tint becomes crimson. 

ROSE PINK is a crude kind of lake of a dull rose-red colour 
prepared from Brazil wood by making a decoction of the wood in 
the usual manner, adding some gypsum and chalk, and then pre- 
cipitating with alumina. 

RED LAKE. In a patent taken out in 1856, Gatty describes 
a process for making a red lake from Brazil wood by adding to 
20 gallons of a decoction (sp. gr. 7 Tw.), 1 gallon of antimony 
chloride at 52 Tw. ; filtering, washing, and drying the pre- 
cipitate. 

Messrs. Roberts & Dale patented in 1857 a process of 
preparing a red lake from barwood. 7 cwts. of barwood are 
boiled in 500 to 600 gallons of water, and 50 Ibs. persulphate 
of tin diluted with water added thereto; the whole is boiled 
for 3 to 4 hours, allowed to settle, and decanted; the lake 
formed is washed, filtered, and dried. 



256 LAKES. 

Barwood is not adapted for preparing lakes, as the colouring 
principle it contains is not soluble in water, and the tin required 
to produce the lake must be added to the wood directly; as a 
consequence, the lake so prepared is apt to be contaminated 
with particles of the wood. 

Another method described in the same patent is to boil bar- 
wood in a solution of 1 to 1^ oz. of carbonate of soda per gallon 
of water, whereby the colouring principle is extracted; to the 
decoction (after straining off the wood) is added sufficient tin 
chloride to precipitate the lake. 

YELLOW LAKES. Dutch Pink, English Pink, and Italian 
Pink, are inappropriate names given to lakes of rather a crude 
kind prepared from quercitron bark. They can be prepared by 
boiling 2 Ibs. of quercitron bark, straining the decoction, and 
adding 1 Ib. of alum ; to the mixture is added 4 Ibs. of fine Paris 
white in small quantities at a time; then the mass is placed on 
one side for 2 or 3 hours, collected, washed, and dried. By 
decreasing the quantity of Paris white, the shade of the yellow 
is made deeper. Lakes from quercitron are used in painting 
theatrical scenery, as they do not lose much of their tint under 
the influence of gas light. 

Another method is to use | Ib. of fustic extract, and 2 Ibs. of 
gypsum, and to precipitate with J Ib. of lead acetate. 

YELLOW LAKE. This is prepared from Persian berries by 
(a) boiling 1 Ib. of the berries with 1 oz. of cream of tartar in 
1 gallon of water, straining, and to the clear decoction adding 
sufficient alum to precipitate the lake, (b) 1 Ib. of the berries 
is boiled with 1 Ib. of alum in water; the liquor is strained, and 
then sufficient carbonate of potash is added to precipitate the 
lake ; care is taken to avoid an excess of the alkali, as this would 
redissolve the colouring matter, (c) A cheap yellow lake, much 
employed in painting scenery, is made by boiling 2J Ibs. of 
Persian berries with 2J Ibs. of turmeric in water for some time, 
then, after straining the decoction, adding to it 1 Ibs. of sul- 
phate of alumina and 6 Ibs. Paris white, allowing the mass to 
stand for some time, filtering, washing, and drying. 

(d) Gatty's Process.- -To 20 gallons of a decoction of quer- 
citron (7 to 8Tw.) is added 1 gallon of antimony chloride at 
52 Tw. The lake is precipitated, washed, and dried. 

ORANGE -CARMINE or ORANGE-LAKE. 1 Ib. of 
Persian berries are boiled in 1 gallon of water and the liquor 
strained; then J Ib. of muriate of tin (commercial stannous 
chloride solution) added and sufficient sodium carbonate to pre- 
cipitate the lake, which is collected, washed, and dried. This 



MADDER-LAKES. 257 

lake has a bright orange colour, and is chiefly used by calico- 
printers. 

Orange-Lake. J Ib. of fustic extract is dissolved in water, and 
^ Ib. of lead acetate added thereto. 

Orange-Lake. Precipitate a mixture of fustic extract and 
annatto dissolved in a little carbonate of soda with alum and 
tin crystals. 

MADDER-LAKES. These are largely used by artists on 
account of the brilliancy of their tint and superior permanency 
over other lakes. Several methods have been published for their 
preparation. 

(a) Englefield's Process. Tie 2 oz. of madder in a piece of 
thin cloth, and beat it well in 1 pint of water in a stone mortar, 
and repeat the process with 5 successive pints of fresh water, 
until the material ceases to yield colour ; the mixed liquors are 
boiled in an earthen vessel, and 1 oz. of alum is dissolved in the 
pint of water, then a solution of J oz. of potassium carbonate 
is slowly added, and the mixture allowed to stand until cold; the 
top liquor is decanted off, and the lake well washed in hot 
water, filtered, and dried. 

(6) Macerate 2 Ibs. of ground madder in 1 gallon of water for 
10 to 15 minutes, and repeat the process with 2 or 3 successive 
quantities of water; the liquors are mixed and J Ib. of alum is 
added; the mixture is gently heated nearly to the boiling point 
for 3 or 4 hours, filtered, and a solution of sodium carbonate 
added as long as a precipitate falls; this is filtered, well washed 
with water, and dried. 

J:) 2 Ibs. of madder are steeped in tepid water for 26 hours, 
a slight fermentation set up, whereby the decomposition 
of the glucoside of the madder is effected, which means an increase 
in the colouring power of the extract ; to the mass is now added 
a solution of 2 Ibs. of alum, the mixture being maintained at 
about 150 F. from 3 to 6 hours, when the liquor is strained and 
precipitated with sodium carbonate as long as a precipitate falls 
down ; the precipitate is filtered, washed, and dried as usual. 

(d) Add to the decoction of madder a small quantity of acetate 
of lead solution, which throws down a brown colouring matter 
which is present in the dyestuff ; after filtering this off, the clear 
liquor is treated with alum and tin as usual. 

(e) Garancine is a more or less purified madder, the useless 
glucoside (or ruberythric acid) of which has been converted into 
useful colouring matter, and may be used in the preparation 
of madder lakes. The process is to treat 1 Ib. of the garancine 
with successive portions of a boiling solution of J Ib. of alum 

17 



258 LAKES. 

in 2 gallons of water, using altogether about 2 gallons; after 
extraction, the liquor is filtered, and allowed to cool, when 
the colouring matter separates out as a nocculent precipitate ; 
this is collected and dissolved in ammonia, and to the solution is 
added alum, or a mixture of alum and tin chloride, sufficient to 
precipitate the colouring matter. The depth of colour of the 
lake depends upon the amount of alum used, provided there be 
sufficient ammonia to precipitate all the alum added; the more 
alum the paler the tint of the lake. 

(f) Persoz Process. 1 Ib. of garancine, and 1 Ib. of sodium 
sulphate are boiled together in 18 pints of water; to the mixture 
is added 1 Ib. of alum, previously dissolved in water, and the 
mass allowed to stand for some time for the alum to extract the 
colouring principle of the garancine ; the mass is next strained, 
and to the clear liquor is added 1 Ib. of lead acetate; lead sul- 
phate is precipitated, and this is filtered off; on boiling the clear 
filtrate the lake formed is collected, washed, and dried. 

MADDER KED LAKES. (a) By combining both madder 
and cochineal bright carmine-red lakes can be prepared. A 
decoction of madder and alum is prepared in the manner de- 
scribed above under madder lakes (c) ; to this is added an am- 
moniacal solution of cochineal (prepared by digesting 1 Ib. of 
cochineal in 4 ozs. of ammonia diluted with its own volume of 
water for a few days) as long as a precipitate falls down. This 
is collected, washed, and dried. 

(b) A cheap red lake can be made by mixing decoctions of 
madder and Brazil wood, adding carbonate of soda, and preci- 
pitating with alum, or alum and tin, in the usual way. The 
recipes for preparing madder and Brazil wood lakes may be 
combined to produce the compound lake. 

Madder only contains the colouring principle, alizarine, in 
comparatively small proportion; with it is associated other 
colouring matters and impurities, which, as a rule, are more 
soluble in boiling than in cold water ; hence it is necessary to 
avoid extracting madder with boiling water, otherwise these 
impurities pass into the extract and injuriously affect the tint 
of the resulting lake. The bulk of the colouring matter exists in 
the madder in the form of a glucoside named ruberythric acid ; 
this, by fermentation, is converted into the colouring principle, 
alizarine. It is well to take advantage of this in the process of 
preparing madder lake by steeping the madder in a little tepid 
water for a day or so, and then extracting the colouring principle 
with alum as described under method (c) above. 

Madder lakes, when pure, are fairly bright in tint, and good as 



VIOLET LAKE. 259 

regards both their covering and colouring powers. They are 
almost entirely soluble in solutions of caustic potash or caustic 
soda, but not in weak ammonia, which character serves as a 
distinguishing point from carmine. Boiled with dilute sulphuric 
or hydrochloric acid madder lakes are decomposed, alizarine, the 
colouring principle of the madder, being liberated as a brownish- 
yellow precipitate. When used as pigments they are more per- 
manent than other lakes ; in dry air they are nearly permanent, 
but in moist air they fade a little. 

GREEN LAKES. Chinese green and sap green are similar 
to lakes in composition and properties ; they have already been 
described. There are no other green pigments prepared from 
vegetable dyestuffs by precipitation as lakes ; most of what are 
sold as green lakes are either lakes made from coal-tar colours 
(as described below) or with mixtures of either Prussian blue or 
indigo and gamboge, not true lakes. 

VIOLET LAKE. (1) A violet lake can be made by adding 
2J gallons of antimony chloride, at 52 Tw., to each 16 gallons of 
a decoction of logwood (10 Tw.). The lake is immediately pre- 
cipitated, and is filtered, washed, and dried. 

(2) 3 Ibs. of alum are dissolved in 1 gallon of water, and to the 
solution is added 2 J Ibs. of lead acetate previously dissolved in a 
little water; the precipitated sulphate of lead is filtered off and the 
clear liquor is used in preparing the lake. 6 Ibs. of logwood are 
made into a decoction with 10 gallons of water, and to 10 gallons 
of this liquor 1 gallon of lead-alum liquor is added ; the lake pre- 
cipitated is collected, washed, and dried. It has a fine violet 
colour, but is not fast. 

The lake colours are often sold in the form of pastes for the 
purpose of colouring beverages, sweets, &c. The author has had 
occasion to make analyses from time to time of such pigments, 
and the following are selections from such analyses : 

ANALYSES OF LAKE COLOURS. 







1. 


2. 


3. 


4. 5. 


6. 




7. 


Water, 


t 


80-40 


80-00 


79-45 


68-2 


79 


45 


82 


o 


88-75 


Ash, . 


t t 


6-75 


9 


55 


10-20 


19-9 


10 


60 


9 


3 


3-00 


Colouring 


matter, 


12-85 


10 


45 


10-35 


11-9 


9 


95 


8 


7 


8-25 



100-00 100-00 100-00 100-0 100-00 loo-o 100-00 

1. Scarlet from Lima wood with alumina. 

2. Orange-scarlet from peach wood with alumina. 

3. A brownish-yellow from Persian berries with alumina. 

4. Olive-yellow from Persian berries with lead and chalk. 

5. Orange made from fustic ~and annatto with alum and tin. 

6. Violet-brown from Lima wood with alum, lead, and iron. 

7. Blue-black from logwood and iron. 



260 LAKES. 

General Considerations on Lake-making. The prepara- 
tion of lakes is a matter of some difficulty, especially when it is 
desired to prepare successive batches, perhaps at some consider- 
able interval of time, of the same depth and brilliance of colour. 
This difficulty arises almost entirely from the variable character 
of the dyestuffs or colouring matter used, the natural colouring 
matters varying considerably in actual colouring power; this 
makes it impossible for the users, either lake-makers or dyers, 
to always work from very definite recipes ; these must be modified 
from time to time, according to the actual strength of the batch 
which is being used. If the colouring power of the dyestuff is 
weak, then more must be used to produce a lake of a given 
shade than if the colouring power is either normal or above the 
average; or, if no more dyestuff is used, then the proportion 
of base must be varied to suit the strength or colouring power 
of the dyestuff. 

The depth of tint or colour of the lake which is prepared 
depends, as may naturally be inferred, on the proportions of the 
base or precipitating agent to the colouring matter ; the greater 
the proportion of base the paler will be the tint of the lake 
produced. As the preparation of lakes is a chemical operation 
depending upon the combination of the colouring principle of the 
dyestuff with the base (alumina, tin, &c.), there will be a definite 
relationship between the two bodies, resulting in the formation 
of a lake having the maximum depth of colour. What these 
proportions are has not yet been determined ; they necessarily vary 
with different colouring matters, and even with different batches 
of the same colouring matters. Any excess of dyestuff used 
over these proportions does not lead to any increase in the depth 
of the lake formed, but such excess is left unused, and may pro- 
bably be thrown away; on the other hand, any excess of base 
reduces the tint of the lake, as has been explained above. 

After precipitation, the lake should be well washed, and dried 
slowly at as low a temperature as possible. Usually lakes are 
made up in the form of conical masses, drops, or troches ; this is 
done by mixing a small quantity of gum or glue water with the 
wet lake, then pressing the mass into moulds, or moulding 
between the fingers, then drying in the usual way. 

The following description of the reactions given by various 
chemical reagents with decoctions of the principal natural dyestuffs 
used in lake-making may be found of use, especially in the 
examination of lakes for the purpose of ascertaining how they 
are made ; but it may be pointed out that it will always be 
advisable for the analyst to make such experiments on his own 



REACTIONS OF NATURAL DYESTUFFS. 261 

account, for the purpose of observing the exact shade of colora- 
tions produced, as it is impossible to convey, with sufficient 
exactitude, an idea of the actual tint of any coloration or 
precipitate which may be produced. 

Cochineal. Alumina sulphate gives a bright crimson solu- 
tion ; on the addition of sodium or potassium carbonate a 
crimson precipitate falls down. Stannous chloride gives a dull 
purplish coloured precipitate ; copper sulphate a purple pre- 
cipitate; ferric chloride a brown precipitate; caustic soda a 
bluish crimson solution. Strong sulphuric acid changes the 
colour of the decoction to an amber colour, and, on adding 
water, a pale yellow solution is obtained. 

Brazil Wood. Alumina sulphate gives a red precipitate. 
Stannous chloride throws down a pale crimson precipitate; lead 
acetate a violet-rose precipitate; ferric chloride a chocolate- 
brown precipitate; copper sulphate a brick-red precipitate; and 
caustic soda forms a crimson solution. Strong sulphuric acid 
changes the colour of the decoction to a red-brown, and on 
diluting this with water a pale amber solution is obtained. 

Persian Berry. Alumina sulphate has no action. On 
further adding ammonia or caustic soda, a brownish-olive pre- 
cipitate falls down, while alkaline carbonates throw down a 
brighter yellow precipitate. Stannous chloride throws down 
an olive-yellow precipitate, lead acetate a yellow-brown preci- 
pitate, and ferric chloride a dark olive-green precipitate. Nitric 
acid forms a red solution. Strong sulphuric acid forms a yellow- 
brown solution, and on adding water a brown precipitate falls 
down. 

Quercitron. Alumina sulphate throws down an ochre- 
yellow precipitate, stannous chloride an orange-yellow precipi- 
tate, lead acetate a dark yellow-brown precipitate, copper 
sulphate a brownish-olive precipitate, and ferric chloride a dark 
olive-green precipitate. Caustic soda forms a dark yellow- 
brown solution, from which acids throw down a yellow-brown 
precipitate. Strong sulphuric acid forms a brownish-yellow 
solution, from which, on adding water, a dark brown precipitate 
falls down. 

Fustic. Alumina sulphate, stannous chloride and lead 
acetate throw down orange-yellow precipitates; the first is 
bright, the last rather duller than those with the other two salts. 
Copper sulphate throws down a dull yellow precipitate, and 
ferric chloride a dark olive-brown precipitate. Strong sulphuric 
acid forms a brown-yellow solution, from which, on diluting 
with water, a brown precipitate falls down. 



262 LAKES. 

Annatto. Alumina sulphate throws down a brownish 
precipitate, stannous chloride an orange-red precipitate, ferric 
chloride a red-brown precipitate, while strong sulphuric acid 
forms a dirty yellow-brown solution. 

Turmeric. Alumina sulphate gives a yellow precipitate, 
ferric chloride a brownish-yellow precipitate, and stannous 
chloride a yellow precipitate. Caustic soda forms an amber 
solution, and strong sulphuric acid a pale yellow solution. 

Logwood. Alumina sulphate changes the colour of the 
decoction to a crimson-red ; ferric chloride throws down a bluish- 
black precipitate; potassium bichromate causes the formation 
of a black gelatinous mass after standing a short time; copper 
sulphate throws down a violet-black precipitate, and antimony 
chloride a violet precipitate; caustic soda changes the colour of 
the decoction to a violet, while hydrochloric acid turns it red. 



ANILINE-LAKES. 

Of late years the use of the coal-tar colours for the preparation 
of lake-pigments has grown very extensively, and now such lake- 
pigments are largely used, especially for paper-staining, manu- 
facture of paper-hangings, printing, lithography, <kc. In many 
cases they have displaced some of the older colours, especially 
such as emerald-green, which are rather poisonous, and vermilion, 
which is expensive. 

The aniline or coal-tar lakes are not true lakes, although they 
are prepared by precipitating a coal-tar colour with a suitable 
precipitating agent ; they always contain another substance in 
addition to the lake proper, which is added for the double purpose 
of increasing the covering power or body of the lake as well as, in 
many cases, of modifying or toning down the shade ; at the same 
time it reduces the cost of the lake and brings it within the 
purchasing power of painters for use in common decorative work. 
Three things are required in the preparation of coal-tar colour 
lake-pigments 1st, the colour; 2nd, the precipitating agent; 
3rd, the base or, as it is sometimes called, the carrier. 

1st. THE COLOURING MATTER. The dyestuffs pre- 
pared from coal-tar products may be divided into 16 groups 
according to their chemical composition and relations ; but for 
the present purpose they may be divided into three groups 
differing from one another in the character of the methods 
used in preparing lakes from them. Although something like 
300 different coal-tar colours are to be found in the market, yet, 



ANILINE-LAKES. 263 

for various reasons which will be pointed out later on, many of 
these cannot be used in the preparation of lake-pigments. 

1. Basic Colours. These dyestuffs are salts of certain com- 
plex organic bases prepared, in the first instance, from aniline, 
toluidine, &c. They include such dyestuffs as magenta, brilliant 
green, auramine, aniline-blue, methyl-violet, Bismarck brown, 
benzoflavine, rhodamine, &c. These colouring matters form, with 
certain weak organic acids, such as tannic acid, picric acid, &c. 
coloured precipitates, and it is on this property that their use in 
the preparation of lake-pigments is based. 

(a) Colouring matters precipitated by tannic acid. All the basic 
coal-tar dyestuffs are precipitated by tannic acid, but the following 
are of special value as lake-pigment makers : magenta, safranine, 
auramine, aniline-blue, brilliant green, methyl-green, new green, 
Nile blue, rhodamine, phosphine, methyl-violet, Paris violet, 
Hofmann violet, Bismarck brown, chrysoidine, quinoline-yellow, 
benzoflavine, pyronine G. The combination between the tannic 
acid and the colouring matter is a chemical one and takes place 
in definite proportions; but, as these dyestuffs are rarely sold 
pure, only approximate quantities can be given in any recipes for 
their transformation into lake-pigments. There is one point to 
be noticed in connection with the precipitation of these basic 
colours with tannic acid, which is that the precipitated lake has 
the property of carrying down with it some of the colouring 
matter in a free form. 

It is best to use tartar emetic along with the tannic acid in 
making the lakes, so as to form a double combination of the 
tannic acid with the colouring matter on the one hand and with the 
antimony of the tartar emetic on the other, which has a material 
influence on the complete precipitation of the dyestuff and on its 
fastness to light and air. 

One fault of the tannic lakes is that when used as oil colours 
they do not dry well or completely; the cause of this lies with 
the tannic acid, which exerts a retarding action on the drying of 
linseed oil, and, if present in large proportions, will almost com- 
pletely prevent the oil from drying. Hence tannic acid lakes 
cannot be used with satisfactory results as oil colours, but for all 
other vehicles they are quite satisfactory. The precipitation of 
the lakes is more complete if sodium acetate be added ; this acts 
by neutralising the hydrochloric acid which is liberated from the 
colour by the action of the tannic acid and which exerts a solvent 
action on the colour lake, the free acetic acid formed from the 
sodium acetate having no such action. The tartar emetic has a 
similar action (see below). 



264 LAKES. 

(5) Colouring matters precipitated by picric acid. This sub- 
group includes auramine, night-blue, methyl-blue, methyl-green, 
brilliant green, safranine, and a few others. Picric acid being 
itself a yellow dyestuff, it is obvious that the shade of the 
resulting colour lake must be modified thereby; thus, the greens 
which are naturally of a blue shade and are so precipitated by 
tannic acid, are thrown down by picric acid of a yellow-green 
tone, while the reds take a yellower tone, and the blues become 
greenish. The picric acid lakes may be used as water-colours, 
but they cannot well be used as oil-colours, for the oil does not 
dry properly; they thus resemble the tannic acid lakes, but, if 
anything, the defect is stronger. 

2. Acid Colouring Matters. The term " acid " as here used 
has special reference to the fact that the colours comprised in this 
group are dyed on to wool or silk from baths containing a small 
quantity of free acid and not to the fact that the colours them- 
selves are of an acid character, although many of them are. 

Some of them are derived from the basic colours by a process 
of sulphonation, by which means they are rendered capable of 
dyeing wool or silk from acid baths, which, previously, they were 
not capable of doing. The great majority of the acid colours are 
what are called azo-colours, that is, bodies characterised by con- 
taining a group of two nitrogen atoms, N = N , combined 
with some organic radicle of a basic or phenolic character; in 
some colouring matters there is more than one such group. 
Other dyestuffs belonging to this group are nitro-derivatives, 
usually of phenols. 

These colouring matters are precipitated from their solutions 
by acids and metallic salts (such as lead acetate, barium chloride, 
alumina sulphate, alum, zinc sulphate, magnesium sulphate, &c.) 

Naturally, they vary much in the degree or ease with which 
they are precipitated. Some are thrown down by all the agents 
above noted ; others, again, are only precipitated by one or two 
of the salts. Others, again, require the solutions to be slightly 
alkaline before they are precipitated. The following lists, 
showing how these acid dyes are precipitated, will illustrate this 
point. 

(a) Colours precipitated ~by lead acetate. Lead acetate will pre- 
cipitate a very large number of the coal-tar colours belonging to 
the acid group; in some cases the precipitated lake is of sufficient 
good colour to be of service in making lake pigments, in other 
cases it is not so. To enter into full details with regard to all 
the colours would take up more room than can well be spared in 
this book, but the following list and the notes appended thereto 



ANILINE-LAKES. 265 

will be found of use : Scarlet BB (lake of a poor shade, not 
serviceable), mandarin G, orange G, orange IV (the lake is of a 
brownish shade, and not useful), fast scarlet 3R (very good 
lake), acid mauve B, yellow N (fine shade of colour), croceine- 
orange, fast red T, alkaline blue, citronine A, tropseoline OO 
(lake of a very pale colour, not useful), citronine O, methyl- 
yellow, new yellow, scarlet G, scarlet R, ponceau 2G, fast violet 
(the lake is but of a poor shade), orchil brown B, fast red A, 
azo-yellow, double brilliant scarlet 2R, croceine 3B, quinoline- 
yellow, crystal scarlet 6R, phosphine, scarlet OO, scarlet GT, 
vermilline scarlet KK, Bordeaux S (very good shade of colour 
lake), all the eosine colours, benzoazurine R, benzoazurine 3G 
(gives rather a poor shade of lake), diamine-blue B, Titan pink 
(gives a dark crimson colour lake), Congo 4R,* chrysamine (gives 
rather a dark brownish yellow lake), and chrysophenine (a fine 
precipitate). 

(b) Colours precipitated by barium, chloride. Scarlet BB, pon- 
ceau, orange G (gives a fine bright colour-lake, very useful), 
orange IV, fast scarlet 3R (a fine bright colour-lake), acid mauve 
B, claret-red B, yellow N, croceine-orange, croceine-scarlet, fast 
red T, alkali-blue, Victoria blue B, citronine O, methyl-yellow, 
new yellow, scarlet G, ponceau 2R, scarlet R, resorcin-yellow, 
fast violet, orchil-brown B, fast red A, azo-yellow, double 
brilliant scarlet 2R, Indian yellow, crocein 3B, quinoline-yellow, 
crystal scarlet CR (not so good a lake as with lead acetate), 
naphthol-yellow S, phosphine, scarlet 2R, scarlet GT, vermilline- 
scarlet KK, scarlet 2RJ (gives a very fine lake), scarlet 3R, ben- 
zoazurine R, benzoazurine G, benzoazurine 3G, diamine-blue B, 
Congo 4R, chrysopbenine (gives a good lake), chrysamine (pre- 
cipitates as a brownish lake). 

(c) Colours precipitated by alumina sulphate Scarlet BB 
(the lake has a brownish tone), mandarin G, Orange IV, fast 
scarlet 3R, croceine-orange, alkali-blue, Victoria blue B, citro- 
nine O, scarlet G, fast violet (but a poor shade of colour-lake), 
orchil-brown B, fast red A (but a poor shade of lake), azo-yellow, 
Indian yellow, scarlet GT, benzoazurine R, benzoazurine G (a 
good precipitate), benzoazurine 3G, Titan pink, Congo 4R, chry- 

* The red colouring matters, which, like Congo-red and benzopurpurine, 
are derived from benzidine, tolidine and stilbene, are precipitated by lead 
acetate and other precipitants, but, in consequence of their being sensitive 
to acids in a great degree, the resulting colour-lake is liable to be much 
affected in colour by acid precipitants. Thus, Congo-red will give a blue 
lake instead of a red, and the others are more or less similarly affected ; 
therefore they are practically useless for making lake-pigments. 



266 LAKES. 

sophenine (the precipitate has an olive-brown shade), chrysamine 
(gives a brown precipitate), and diamine-blue B (a fine shade). 
Sulphate of alumina does not precipitate so many colouring 
matters as lead acetate or barium chloride, probably on account 
of its great acidity, which keeps the colour-lakes in solution to a 
large extent; by neutralising this acidity with soda or ammonia 
some colours can be precipitated, but the addition of alkali has 
to be made with great care, or the shade of the colour-lake 
may be affected. Freshly-precipitated and washed hydroxide 
of alumina, A1 2 H 6 6 , has a strong affinity for many of the 
coal-tar colours of all classes, and this may (as will be shown 
presently) be taken advantage of in the preparation of lake- 
pigments. 

The remarks appended to group a regarding the so-called Congo- 
colours, are applicable to the precipitates obtained with barium 
chloride and alumina sulphate. 

These lists do not pretend to be exhaustive, but they include 
all the colouring matters which are most useful for this particular 
purpose. It is well to point out that the same name is given by 
different makers to different dyestuffs, so that it is quite possible 
for, say, the scarlet G- of one maker to give a precipitate with 
alumina sulphate, while the scarlet G of another maker will not 
give a precipitate. 

Of the three precipitating agents, barium chloride gives the 
best and most satisfactory results; although both lead acetate 
and alumina sulphate give useful colour-lakes; but barium 
chloride is a more universal precipitant than either of the 
others, and the colour-lakes it gives are not affected by admixture 
with sulphur pigments, as is the case with lead acetate colour- 
lakes, which are apt to go brownish if mixed with pigments 
containing sulphur or if exposed to the action of sulphur gases. 

Lake-pigments made with either barium chloride, lead acetate, 
or alumina sulphate, are very satisfactory in use, and are service- 
able either as oil- or water-colours. 

3. Adjective Colouring Matters. This class of colouring 
matters was named by Bancroft adjective dyestuffs; their pecu- 
liarity is that they are usually not coloured of themselves, and, used 
alone, are not capable of imparting any colour to textile fibres, but 
that they require the aid of a second substance, which, in dyeing, 
is called the mordant, to develop the colour and to fix it on the 
fibre; further than that the colour which is so fixed on the fibre 
is largely influenced by the character of the mordant, and with 
different mordants different colours are produced; thus alizarine, 
with alumina gives a bright red, with iron a violet, with tin. 



PRECIPITATING AGENT FOR ANILINE-LAKES. 267 

a scarlet, and so on. The dyestuffs belonging to this group 
are capable of yielding lake-pigments, which are characterised by 
their permanence. They are rather difficult to prepare in any- 
thing like brilliant hues. 

For fuller information relating to the coal-tar colours, the 
student may consult Benedikt and Knecht's " Chemistry of the 
Coal-tar Colours," or the author's "Dictionary of the Coal-tar 
Colours." 

2nd. THE PRECIPITATING AGENT. The nature of the 
precipitating agents used in preparing lake-pigments has been 
partly considered while dealing with the coal-tar colours them- 
selves, and very little more needs to be said. Tannic acid is the 
precipitant for the basic colours. This should be bought of good 
quality; in fact, for this particular purpose the purer it is the 
better; the common qualities of tannic acid are more or less 
adulterated with dextrine, <fec., and are slightly coloured, which 
qualities have an injurious influence on the brilliancy of the 
pigment thrown down by it. In addition to tannic acid, tartar 
emetic is used; this adds considerably to the permanence of the 
pigment, probably for two reasons; one is that the antimony of 
the tartar emetic combines with the tannic acid to form an in- 
soluble tannate of antimony, and with this the colouring matter 
combines, thus forming a more insoluble and therefore more per- 
manent pigment; then, again, most of the colouring matters are 
sold in the form of hydrochlorides of the colour-base, and in the 
process of making the lake the hydrochloric acid is set free, and 
may tend to prevent proper precipitation of the lake. When tartar 
emetic is used the hydrochloric acid acts upon this and combines 
with the alkali, liberating tartaric acid, which has no action on 
the colour-lake formed. The addition of sodium acetate has been 
proposed for the purpose of preventing this formation of free 
hydrochloric acid. 

As regards the precipitating agents used with the acid dye- 
stuffs, viz., barium chloride, lead acetate, alumina sulphate, &c., 
these should be of good quality, and contain neither insoluble 
matter nor free acid ; this last remark applies more particularly 
to alumina sulphate, which is apt to contain free acid, whereby 
the formation of the colour-lake is prevented; in using this agent 
the addition of sodium acetate to neutralise any free sulphuric 
acid which may be present, or which may be formed in the pro- 
cess of precipitation, is to be recommended. 

For the adjective dyestuffs the acetates of alumina, chrome, 
&c.j will be found to give good results. Many processes for the 
preparation of pigments from these colouring matters are based 



268 LAKES. 

on the formation of the hydroxides by precipitation with sodium 
carbonate or sodium hydroxide, and then combining this with 
the colouring matter. This is not a satisfactory method, because 
the alkali has a tendency to affect the shade. 

3rd. THE BASE. The base or carrier exerts a most impor- 
tant influence on the value of the lake as a pigment ; the body or 
covering power almost entirely depends upon the base, while this 
also modifies the tint or shade of the pigment very greatly. The 
base used is commonly one or other of the white pigments (which 
have already been described), but in one or two lake-pigments 
other bases, such as red- or orange-lead, is present. 

The following bases are used in making lake-pigments : 
barytes, whiting, china clay, gypsum, French chalk, zinc-white. 
For details as to the composition, &c., of these reference is made 
to the chapter on white pigments. 

Barytes is the base most commonly used; the common 
qualities are apt to be gritty, a fault which must be avoided ; 
further, in making what are called pulp-colours for paper-stainers, 
barytes is not very useful, as its absorbent properties for water 
are not sufficiently strong ; these colours are required to contain 
about 50 per cent, of water, while barytes will not take up more 
than 25 per cent. The artificial barytes, blanc Jlxe, is a good 
base, and the method of preparing the pigment may be so devised 
as to cause the formation of this form of barytes during the pro- 
cess of making the lake, and then a pigment of great brilliancy 
of hue and covering power is obtained; but this method of work- 
ing is more costly than using the natural barytes. 

Gypsum forms a very good base for lakes. It is lighter than 
barytes; hence it does not make the pigment feel so heavy, and 
has less tendency to separate by subsidence when made into a 
paint; it takes up rather more water than barytes, and is there- 
fore better for pulp -colours. In covering power it is about equal 
to barytes. Precipitated calcium sulphate may now be obtained 
as a bye-product in the manufacture of many chemical products ; 
this form would be found useful as a base, and better, in fact, 
than the natural gypsum. Gypsum, weight for weight, takes 
rather more colouring matter to produce a given shade of lake 
than does barytes. 

China clay makes a good base for these lake-pigments, being 
quite inert in all its properties, as well as of good covering 
power and colour. It is largely employed in making pulp- 
colours, owing to its great absorbent properties for water, in 
which respect it is superior to either barytes or gypsum ; on the 
other hand, unless a large proportion of clyestuff is used the 



LAKES FROM COAL-TAR COLOURS. 269 

resulting lake is apt to appear chalky, on which account china 
clay is not suitable for making pale-tinted lakes. 

French chalk would make a good base, so far as its chemical 
properties are concerned ; but it does not work well as a paint, 
being apt to be slimy. 

Whiting is a fairly good base for lakes, and is largely used for 
this purpose ; although, owing to its somewhat alkaline proper- 
ties, it does not suit all colouring matters. Like china clay, it is 
liable to make the lake appear chalky in tone, and, therefore, can 
only be used in making dark tints. It does not suit lakes made 
from basic colours, owing to the action of tannic acid on it. 

Zinc-white makes a good lake, but its cost is against its 
meeting with an extensive use for this purpose. 

To produce a given shade of lake, barytes takes less colouring 
matter than any of the other bases noted, and, consequently, a 
barytes-lake costs a trifle less than does a lake with other bases ; 
next to barytes comes gypsum, which takes from one-and-a-half 
to twice as much dyestuff in proportion to barytes, while china 
clay takes rather more than twice as much. Pulp-colours, such 
as are used by paper-stainers, are made so as to contain about 
50 per cent, of water, and, in making these, barytes cannot well 
be used, as it will only take up about 25 per cent.; the best base 
for such is a mixture of barytes, china clay, and gypsum, in 
about equal proportions. 

In any case the base should be of a good white colour, and free 
from any trace of grit. Before using, it should be ground with 
water, and then sieved, so as to obtain it of as fine a consistence 
as possible. 

PROCESS OP MANUFACTURE. The method of making 
pigments from the coal-tar colours is comparatively simple, and 
does not necessitate the use of any special plant; the only 
requisites are tanks for dissolving the various colouring matters 
and precipitating agents, and a tank (like those shown in Fig. 18, 
p. 118) for precipitating the lake in. 

The colouring matter is dissolved in a vat or tub, by simply 
mixing the dyestuff with water. In the case of those colouring 
matters which, like some brands of violets and magenta, are sold 
more or less impure, and in a cake form, it is best to dissolve 
them in boiling water. A good proportion is 1 Ib. of colouring 
matter to 10 gallons of water. It is advisable to strain the 
solution before using it, so as to free it from particles of 
undissolved dyestuff and grit. 

The precipitating agent is dissolved in a separate vat and the 
solution filtered to free it from the dirt and other insoluble 
matter which would deteriorate the lake. 



270 



LAKES. 



Into a third vat is placed a quantity of water, which is heated 
to from 120 to 150 P., and then the base (barytes, china clay, or 
whatever is used) is thoroughly mixed or diffused through the 
water, special care being taken to break down all lumps, because 
these would give the lake a speckled appearance, a thing which 
is to be avoided. Next, the solution of the colouring matter is 
run in, thoroughly mixed with the base, and, after heating the 
whole to the temperatures given, the precipitating agent (in solu- 
tion) is run in slowly with thorough agitation ; when all has been 
run in, the lake formed is allowed to settle, and the top liquor 
run off; this should be colourless, or nearly so ; if it be strongly 
coloured, the precipitation of colouring matter has not been com- 
plete and more precipitant is added. The lake is now washed by 
adding clean water and then finished, as may be required, by the 
usual methods. When required for pulp-colours the lake simply 
requires filtering off; if required to be dried then, after filtering, 
it must be dried in the stove at a temperature below that by 
which the colour or tint of the lake would be affected. 

The process here given is generally applicable to all the coal- 
tar colours, but, in some cases, modification of the details are 
required, which will be pointed out when dealing with the 
pigments themselves. 

LAKES FROM BASIC COAL-TAR COLOURS. 
Magenta lake. 100 Ibs. of barytes, 1 Ib. of magenta, 1J Ibs. 
of tartar emetic, and 1| Ibs. of tannic acid make a lake of a 
deep crimson colour. 

Bluish-pink lake. 100 Ibs. of barytes, 3 Ibs. of rhodamine. 
3 Ibs. of tartar emetic, and 3 Ibs. of tannic acid. This makes a 
lake of a peculiar shade of bluish-pink, which is fairly resistant 
to exposure to light and air. 

Pale crimson lake. 100 Ibs. of barytes, 2 Ibs. of safranine 
prima, 2 Ibs. of tartar emetic, and 3 Ibs. of tannic acid. The 
lake obtained is a fine shade of crimson. 

Violet lake. 100 Ibs. of barytes, 1 Ib. of aniline-violet, 1 Ib. 
of tartar emetic, and li Ibs. of tannic acid. The shade of this 
lake will depend entirely upon the shade of the violet used, 
which may vary from a violet-red (violet 3 R) to a pure violet 
(violet 5 B). Either methyl-violet, or Hofmann's violet, or Paris 
violet may be used. 

Blue-green lake. 100 Ibs. of barytes, 1 Ib. of brilliant green, 
1 Ib. of tartar emetic, and 1J Ibs. of tannic acid. This gives a 
very deep bluish-green lake. 

Yellow-green lake. 100 Ibs. of barytes, 1 Ib. of brilliant- 
green, J Ib. of auramine, 1J Ibs. of tartar emetic, and 2J Ibs. of 



LAKES FROM ACID COAL-TAR COLOURS. 271 

tannic acid. This gives a very nice yellow-green lake; by varying 
the proportions of the two dyestuffs a great variety of green lakes 
can be made and also a very good imitation of emerald-green may 
be obtained by their means. 

Yellow lake. 100 Ibs. of barytes, 3 Ibs. of auramine, 3 Ibs. of 
tartar emetic, and 4 Ibs. of tannic acid. This lake is very good 
and a tolerably permanent one. 

Orange lake. 100 Ibs. of barytes, 2 Ibs. of chrysoidine, 4 Ibs. 
of tannic acid, and 2 Ibs. of tartar emetic. 

Brown lake. 100 Ibs. of barytes, 2 Ibs. of Bismarck brown, 
2 Ibs. of tartar emetic, and 4 Ibs. of tannic acid. 

In making all these lakes the barytes is mixed with water and 
the tartar emetic is placed in the same tub or vat, the colouring 
matter added, and the tannic acid (previously dissolved in another 
tub) run in slowly ; after which, the lake is finished as described 
above. In the place of barytes any other of the above-mentioned 
bases can be used. By using mixtures of the dyestuffs in various 
proportions, as, for instance, safranine and auramine, brilliant 
green and Bismarck brown, a great variety of lakes of various 
colours can be prepared. 

Another method of preparing lakes from basic coal-tar colours 
is to use picric acid, as in the following case : 

Yellow-green lake. 100 Ibs. of barytes, 1 Ib. of brilliant 
green, and 1 Ib. of picric acid. The barytes and green are 
diffused through water as usual and, when ready, the picric acid 
(previously dissolved in water) is run in ; the lake precipitated 
is finished as usual. 

Another process of making lakes from basic coal-tar colours 
consists in precipitating them with resinates of alumina or mag- 
nesia. This is carried out as follows : 100 Ibs. of rosin are 
dissolved by boiling in a solution of 10 Ibs. of 77 per cent, caustic 
soda, and of 33 Ibs. of soda crystals in 100 gallons of water ; 
when the rosin has completely dissolved, 5 to 15 Ibs. of the dyestuff 
are added, the quality used being varied according to the depth 
of colour of lake required. Then a solution of magnesium or 
alumina sulphates are added in slight excess ; the lake precipi- 
tated is collected, washed, and dried. These resinate lakes are 
soluble in alcohol, benzene, chloroform, ether, &c., and are, there- 
fore, used for colouring varnishes. 

LAKES PROM ACID COAL-TAR COLOURS. The 
number of acid coal-tar colours, especially the reds, is very great, 
and it is impossible in this work to enumerate and mention what 
lakes can be made from these dyestuffs; the examples given will, 
however, serve to illustrate the principles on which the conver- 



272 LAKES. 

sion of acid coal-tar colours into pigments are based, and enable 
any intelligent colour-maker to apply them in producing a lake 
from any colouring matter belonging to this group. 

Among the lakes of this group are the vermilionettes and 
royal reds. 

VERMILIONETTES AND ROYAL REDS. Vermilion- 
ettes were introduced to this country some twelve or fourteen 
years ago by the Silicate Paint Company of Liverpool, as a 
substitute for vermilion, which they have displaced to a con- 
siderable extent. Royal reds were introduced by Messrs. 
J. B. Freeman & Company of London. These pigments, and 
some others, sold under a variety of fancy names (such as 
Victoria reds, signal-reds, &c.) owe their brilliant colour to 
eosine ; the deeper shades of the vermilionettes and the royal 
reds also contain orange lead; this is used for two reasons, one 
is to make the pigment heavy, and a closer imitation of ver- 
milion, the other is to increase the depth of colour and the body. 

These two pigments are made in a great variety of tints, from 
a very pale pinkish-red to a very deep scarlet; most makers keep 
several shades of both in stock. Vermilionettes are now made 
from barytes and eosine, with a precipitating agent, although, 
when first introduced, they contained orange lead as well, and 
some makes do so now ; royal reds contain both barytes and 
orange lead as well as the eosine. The precipitating agent most 
used is lead acetate, although alum is also used occasionally. 

The tint and brilliancy of these pigments depend largely 
upon the kind of eosine used. There are several varieties of 
eosines made, of which the following are the principal. They 
are all very bright scarlet dyestuffs, and may be divided into 
blue-shade eosines and yellow-shade eosines, distinguished by 
the marks B or BN for the former, and by J or G for the latter; 
these being the most useful for vermilionette-making : 

Eosine A, Eosine J, Eosine GG, is usually the potassium salt 
of tetrabromfluorescein, C 20 H 6 Br 4 O 5 K 2 , and of a yellow shade. 

Eosine BN, Safrosin, is the potassium salt of dibromodinitro- 
fluorescein, C 20 H 6 Br 2 N 2 O 9 K 2 , and belongs to the blue-shade 
eosines. 

Erythrosine, Eosine J, is the potassium salt of tetraiod- 
fluorescein, C 20 H 6 I 4 5 K 2 ; it gives very blue shades of ver- 
milionettes. 

Phloxine, Erythrosine B, and Phloxine T are bromo-chloro- 
derivatives of fluorescein, and yield blue shades of vermilionettes. 

Rose Bengale is a very fine blue-shade eosine ; chemically, 
it is the potassium salt of tetraiod - dichlor - fluorescein, 
^20 H 6 C1 2 I 4 O 5 K 2 . 



VERMILIONKTTES AND ROYAL REDS. '273 

To prepare vermilionettes or royal reds the method generally 
pursued is to diffuse the base, which may be either barytes or 
orange lead, or both, through water, to add the required quantity 
of eosine, and to heat the mass to about 1 60 F. ; then there is 
slowly added, with constant stirring, a solution of lead acetate 
until all the eosine is precipitated out ; lastly, the pigment is 
allowed to settle, and is finished in the usual way. 

The following recipes for preparing several shades of these 
pigments exemplify the quantities of the materials used : 

VERMILIONETTES. 

No. 1 Pale. No. 2 Deep. No. 3 Deep. 
Lbs. Lbs. Lbs. 

100 100 
30 

6 8 

16 20 



Barytes, . 
Orange lead, 
Eosine, 


100 
30 
2 


Lead acetate, . 


6 




ROYAL REDS. 




No. 1. Pale. 


Orange lead, 


100 Ibs. 
4 ,, 


Lead acetate, 


. . . 10 



No. 2 Deep. 

100 Ibs. 

8 
20 

The tint, as already noticed, will depend upon the chemical 
composition of the eosine which is used, while the depth of colour 
of the red will depend largely upon the quality of the make of 
eosine as well as upon the quantity of dyestuff used. By regu- 
lating the proportion of base to dyestuff many shades of ver- 
milioiiettes and royal reds may be obtained, but, inasmuch as the 
eosine prepared by one maker greatly differs in quality from that 
of another maker, although professedly of the same chemical 
composition, it follows that the same shade of pigment may not 
be obtained from two makers' cosines. The proportion of lead 
acetate which is required to completely precipitate the eosine will 
also vary witli the make of eosine used; it ranges from 2| to 
4 times the weight of the dyestuff; colour-makers should always 
ascertain by a preliminary trial the precise amount required 
for any particular sample of eosine. When the pigment is 
properly made all the eosine is precipitated, and the wash waters 
are quite, or almost, colourless. 

Instead of lead acetate, alum or alumina sulphate may be used; 
but the author considers that these are weaker precipitants and 
that they yield a pigment of inferior hue and permanence. The 
following recipes will show how these two salts may be used : 

18 



274 LAKES. 

VERMILIONETTES. 

No. 1. Pale. No. 2. Medium. No. 3. Deep. 

Barytes, . 100 Ibs. 100 Ibs. 100 Ibs. 

Eosine, . 1 Ib. 14 Ib. 3 

Alumina sulphate, 3 Ibs. Alum, 10 Ibs. Alum, 20 

A very fine pigment may be made as follows: 50 Ibs. of 
barium chloride, 3 Ibs. of eosine, and 50 Ibs. of alumina sulphate 
are dissolved separately in water ; the eosine solution is run 
into the precipitating vat, and then the other two solutions are 
run in simultaneously ; the verniilionette precipitates almost 
immediately, and is finished in the usual way. 

Properties of Vermilionettes and Royal Reds. As pig- 
ments they are very brilliant in hue and have a good depth of 
colour, whether used as oil-colours or as water-colours, as well as 
good body or covering power. For colouring spirit-varnishes 
they are not good, as the eosine they contain is somewhat soluble 
in the spirit, and, consequently, the pigment loses its brilliant 
colour. One fault which they may have is that of blooming, due 
to the solubility of the colouring matter in the vehicle; this 
generally happens when insufficient lead acetate has been used 
to precipitate the eosine; in such cases the pigment usually con- 
tains some traces of free eosine, which passes into the vehicle 
and causes blooming. 

They do not resist lengthened exposure to light and air; hence 
for work which must have permanence they cannot be recom- 
mended; still they will, if well made, stand a good deal of 
exposure, more especially if protected by a coat of varnish. 

Vermilionettes and royal reds may be recognised by their 
greater or less solubility in alcohol, and by the solution 
showing a fluorescence, the character and colour of which will 
depend upon the particular eosine used in their preparation. 
Heat destroys the colour ; if eosine and bary tes alone have been 
used in preparing the pigment then the colour will be completely 
destroyed; but if orange lead has also been used then the residue 
will have a red colour. Nitric acid destroys the colour of these 
pigments. When orange lead is absent, then the colour is 
destroyed completely and at once ; but if it is present, then the 
colour becomes darker and only disappears on boiling, and lead 
may be recognised in the solution. 

SCARLET LAKES. The number of scarlet and red azo- 
olouring matters is very great, and most, if not all, of them can be 
made to yield pigments. It is practically impossible to deal with 
all these, but the recipes which are given will serve to show the 



SCARLET LAKES. 275 

lines on which to work to transform the azo- and acid reds into 
lake-pigments. 

The azo-scarlets are sold under such names as scarlet G-, 
scarlet R, scarlet 2 R, scarlet 3 R, scarlet 2 R J, Ponceau R, 
double brilliant scarlet G, crystal scarlet 6 R, <fcc. ; some of 
these names are common to several makers, but it does not follow 
that the scarlet G of one maker is the same product as the 
scarlet G of another maker ; hence it may happen that while the 
one dyestuff will be precipitated by alumina sulphate, the other 
may not be. 

All the azo- and acid reds are not available for making lake 
pigments ; some yield very good lakes, others very poor ones. 
A few preliminary trials will show colour makers which are and 
which are not useful for this purpose. 

For transforming the azo-colours, whether reds or yellows or 
oranges or other colours, four methods may be used; one or more 
recipes illustrative of each method will be given, and from them 
the colour maker can readily see how to make other pigments 
than those here mentioned. 

1st Method. General method of making given on p. 269. 

Bluish Scarlet. 100 Ibs. of base, 3 Ibs. of scarlet 2 It J, and 
10 Ibs. of barium chloride. 

Scarlet. 100 Ibs. of base, 5 Ibs. of scarlet G, 20 Ibs. of lead 
acetate. 

Scarlet. 100 Ibs. of base, 3^ Ibs. of croceine scarlet M, 10 Ibs. 
of lead acetate. A little ammonia completes the precipitation. 

Deep Crimson, 100 Ibs. of base, 20 Ibs. of amaranth, 60 Ibs. of 
barium chloride. A little addition of sodium carbonate com- 
pletes the precipitation. 

2nd Method. Mix in the precipitating vat J Ib. of eosine A, 
5 Ibs. of croceine scarlet M, and 33 Ibs. of sodium sulphate 
(Glauber's salt). In separate vats dissolve 25 Ibs. of barium 
chloride and 16 Ibs. of lead acetate; when ready, run the barium 
chloride solution into the colour mixture, and then run in the 
lead solution. A very bright scarlet lake is thus obtained. 
This process may be applied to the preparation of lakes from two 
dyestuffs which require different precipitants. 

3rd Method. Very fine pigments can be made by the 
following process, which consists in dissolving in the precipi- 
tating vat 62 Ibs. of Glauber's salt and 10 Ibs. of scarlet R; into 
this solution is run a solution of 70 Ibs. of barium chloride; the 
lake precipitated is finished in the usual way. By substituting 
for the scarlet R any other dyestuff which is precipitated by 
barium chloride other colours of lake pigments can be made. 



276 LAKES. 

s 

Methods 2 and 3 yield very fine lake pigments, but they are 
more costly to make than those made by the general method. 

4th Method. 100 Ibs. of alumina sulphate are mixed with a 
solution of 25 Ibs. of Ponceau E,R, and the whole heated to about 
60 C., when a solution of 100 Ibs. of barium chloride is added; 
the whole is boiled and then allowed to cool down a little, when 
a solution of 60 Ibs. of soda crystals is added. Care must be 
taken in making this addition, as excess of alkali will adversely 
affect the shade of the resulting pigment. The barium chloride 
and alumina sulphate react with one another, forming barium 
sulphate, which acts as the base of the pigment, and aluminium 
chloride, which remains in solution ; this is precipitated on 
addition of the alkali in the form of alumina hydroxide, which 
combines with the dyestuff, thus forming the lake. 

This method may be used with all colouring matters precipi- 
tated by alumina sulphate, and sometimes with others which 
are not so precipitated. 

This process is not quite so good in its results as the second 
or third methods described above. 

ORANGE LAKES. Any of the azo- or acid oranges may 
be used to make lakes by any of the methods described under 
the reds. The following are three recipes for making orange 
lakes: 

1. 1 00 Ibs. of base, 5 Ibs. of orange G, 5 Ibs. of barium chloride. 

2. 100 Ibs. of base, 3 Ibs. of croceine orange, 4 Ibs. of barium 
chloride, used according to the first method. 

3. 62 Ibs. of Glauber's salt, 10 Ibs. of orange G, 70 Ibs. of 
barium chloride, used according to method 3. 

YELLOW LAKES. The following recipes show the method 
of making yellow lakes from some of the azo- and acid yellows : 

1. 100 Ibs. of base, 3 Ibs. of yellow N, 10 Ibs. of lead acetate. 

2. 100 Ibs. of base, 3 Ibs. of Indian yellow, 5 Ibs. of barium 
chloride, used according to method 1. 

3. 62 Ibs. of Glauber's salt, 10 Ibs. of Indian yellow, and 70 
Ibs. of barium chloride, used according to method 3. 

BLUE LAKES. 1. 100 Ibs. of base, 2 Ibs. of Victoria blue R, 
5 Ibs. of barium chloride. 

2. 100 Ibs. of base, 2 Ibs. of alkaline blue, 5 Ibs. of barium 
chloride. 

3. 100 Ibs. of base, 3 Ibs. of alkaline blue, 4 Ibs. of lead acetate. 
There are many shades of alkaline blue, ranging from a very 

red shade 4R, to a very blue shade 6B, so that a great variety 
of tints or shades of lakes can be made; the proportions of 
colouring matter and precipitant given are only approximate, 



GREEN LAKE. 277 

and necessarily they will vary with the kind and make of alkaline 
blue used. 

These recipes are used according to method 1, given under the 
red lakes. 

4. 62 Ibs. of Glauber's salt, 2 Ibs. of alkaline blue, 70 Ibs. of 
barium chloride, used according to method 3, described under 
red lakes. 

BROWN LAKES. 1. 100 Ibs. of base, 3 Ibs. of orchil 
brown B, 6 Ibs. of barium chloride. 

2. 100 Ibs. of base, 10 Ibs. of cotton brown A, 20 Ibs. of barium 
chloride. Both used according to method 1. 

3. 62 Ibs. of Glauber's salt, 10 Ibs. of cotton brown A, 85 Ibs. 
of barium chloride, used according to method 3. This gives a 
dark reddish shade of brown lake. 

VIOLET LAKES. 1. 100 Ibs. of base, 3 Ibs. of acid mauve B, 
1-5 Ibs. of barium chloride. This gives a red shade of violet lake. 

2. 100 Ibs. of base, 3 Ibs. of acid violet 6B, 10 Ibs. of barium 
chloride. This gives a blue shade of violet lake. 

Like the alkaline blues and the basic violets, the acid violets 
are made in a variety of shades, from a red 3R to a blue 6B, so 
that quite a large range of violet lakes, from a red to a violet 
hue, can be made from the acid violets. 

Both these recipes are to be used as described under method 
1 of red lakes. 

3. 62 Ibs. of Glauber's salt, 2 Ibs. of acid violet 3B, 72 Ibs. of 
barium chloride, used according to the third method. 

BLACK LAKE. 100 Ibs. of base, 10 Ibs. of naphthol black B, 
15 Ibs. of barium chloride. This gives a rather grey shade of 
black. 

GREEN LAKE. 100 Ibs. of base, 5 Ibs. of naphthol green B, 
40 Ibs. of lead acetate ; the addition of a little ammonia completes 
the precipitation ; the lake obtained is of an olive-green shade. 

It is obvious that, in all the above recipes for making lakes 
from the acid coal-tar colours, by varying the proportions between 
the base and the colouring matter, a great variety of tints can 
be obtained from the same dyestuff; still, it is advisable not to 
reduce the proportion of dyestuff too much or the resulting lake 
will have too chalky an appearance. In this connection it may 
be mentioned that barytes makes the best pale-tinted lakes; 
china clay cannot be used very satisfactorily for this purpose, 
as, unless a large proportion of dyestuff is used along with it, 
it is apt to make chalky-looking lake-pigments. Necessarily, 
if the proportion of dyestuff to base be reduced, the quantity of 
precipitant required will also be less; the quantities of these 



278 LAKES. 

given in all the above recipes should only be taken as approxi- 
mate ; each particular sample of dyestuff will take its own pro- 
portion of precipitant, and as the actual quality of a dyestuff 
varies with different makers, it is scarcely possible to give 
very exact proportions, unless, of course, the maker's name 
were inserted, and this it is not deemed desirable to do in this 
book. 



LAKES FROM ADJECTIVE COAL-TAR COLOURING^ 
MATTERS. 

The third group of coal-tar colours are called the adjective 
or sometimes the mordant dyeing, dyestuffs, from the fact that, 
as already pointed out, they require the aid of a mordant, as it 
is called, to properly develop and fix the colour; in the most 
representative members of this group, alizarine, nitrosoresorcin, 
gainbine, &c., the colour which is formed varies with the mordant 
used (see p. 250). Most, if not all, the dyestuffs which belong 
to this group possess acid properties, and have the property of 
combining with metallic oxides, like those of alumina, iron, 
chrome, tin, lead, <fcc., to form compounds which are more or less 
coloured, and quite insoluble in water j it is on this property 
that their value in dyeing, calico-printing, or in lake-making 
depends. In the two former arts the colour lake resulting from 
the combination of the dyestuff with the metallic oxide is formed 
on the fibre, while in the last it is formed in the free condition 
or on a base of some kind. The general principle which under- 
lies the various processes for the preparation of lake pigments 
from this group of dyestuffs is that of bringing the dyestuff into 
contact with the oxide of the metal with which it is desired to 
combine it, and thus cause the colour lake to be formed. 

The principal colouring matters of this group are alizarine and 
purpurine ; these two bodies are sold under the general name of 
alizarine, which is made in several brands distinguished by letters, 
AB, SX, V, G, <fec. Practically, there are two kinds of alizarine - 
yellow alizarine, which, with alumina, gives scarlet reds, and 
consists principally of alizarine ; the other kind is the blue aliza- 
rine, which, with alumina, gives more crimson reds than the last, 
and consists mostly of purpurine. Then there are alizarine blue, 
alizarine yellow, alizarine cyanine, gallein, gallocyanine, gallo- 
flavine, nitrosoresorcin, gambine, &c. 

ALIZARINE LAKES. Various processes can be employed 
for converting alizarine into lakes. 

1. Pure Alizarine Lake. Mix 20 ozs. of ordinary commercial 



ALIZARINE LAKES. 279 

alizarine* with 1^ galls, of water; then add 10 ozs. of alumina 
sulphate previously dissolved in water, and 2 ozs. of calcium 
acetate dissolved in water ; boil the whole together for about an 
hour; then add 10 ozs. of soda crystals dissolved in water in 
small quantities at a time, at intervals long enough to allow of 
the subsidence of the effervescence thus set up. The whole mass 
is now boiled for about an hour, then allowed to stand for 24 
hours, filtered, washed, and dried. This makes a dark red lake 
of good body and staining power. The shade or tint will depend 
upon the kind of alizarine used. Excess of soda crystals should 
be avoided, as it causes the lake to be of a dark colour. 

2. Dark Bed Alizarine Lake. Diffuse 100 Ibs. of barytes 
through 50 gallons of water, add 20 Ibs. of alizarine, 10 Ibs. of 
alumina sulphate, and 2 parts of calcium acetate ; stir well 
together, and then allow to stand for two or three hours, stirring 
at intervals to keep the ingredients well mixed. Heat slowly, 
so as to take about two hours to reach the boiling point, and, at 
intervals, add portions of a solution of 10 Ibs. of soda crystals. 
Much of the beauty of the resulting lake depends on the care 
exercised during this stage of the process; too rapid heating, and 
too rapid addition of the soda crystals, has a tendency to cause 
the shade of the lake to be darker than it should be. The lake 
obtained by this process is of a fine ruby red colour. After it is 
formed, as described above, it is finished in the usual way. 

3. Alizarine Lake. Diffuse 100 Ibs. of barytes through 
50 gallons of water, add 10 Ibs. of oleine or Turkey-red oil, 
and boil for one hour ; allow to stand for twenty-four hours, 
stirring up at intervals, add 20 Ibs. of alumina acetate (12 Tw.), 
and 2 Ibs. of calcium acetate, stir all well together, and allow to 
stand for two days ; then boil well for two hours, adding at the 
same time 5 Ibs. of soda crystals. When the lake has formed, 
filter off, wash, and dry. 

4. Alizarine Red Lake. 6| Ibs. of alumina sulphate are dis- 
solved in water, and to this solution one of 1 Ib. of calcium 
chloride is added ; a precipitate of calcium sulphate is obtained, 
but, before this has had time to settle out, a solution of 4J Ibs. of 
soda crystals is added ; the precipitate of alumina and calcium 
sulphate thus obtained is collected on a filter and washed. It is 

* Nearly all the so-called alizarine colours are sold in the form of a paste, 
containing about 20 -per cent, of actual colouring matter. The reason for 
this form is that, as a rule, these dyestuffs are insoluble in water, and it 
has been found by practical experience that if sold in a dry form that the 
dyestuff does not mix well with water, and that they thus give rise to un- 
even dyeing, while the paste form mixes very well with water, and uneven 
dyeing rarely occurs. 



280 LAKES. 

then introduced into a solution containing 3 Ibs. of alizarine, 
1 Ib. of Turkey-red oil, and 1^ oz. of tannic acid, and the mixture 
heated for half an hour to about 70 C., when it will be found 
that the alumina has taken up all the alizarine, and become 
dyed thereby. It is now boiled for one hour longer, and then 
finished in the usual way. 

5. Alizarine Red Lake. Muller Jacobs has patented, in 
Germany, the following process for making an alizarine lake: 
50 grammes of alizarine oil are dissolved in 1,400 cc. of water, 
15 grammes of alizarine, and 0-2 grammes of tannic acid; the 
mixture is heated to boiling, when 60 cc. of a solution of 
alumina sulphate of 1'1014 (20'3 Tw.), specific gravity, which 
has been previously mixed with 22 per cent, of soda crystals, are 
added. The lake soon forms, especially on boiling for some time; 
it contains some oil which can be extracted with ether. The 
use of such a large proportion of oil is objectionable, as it 
makes the resulting lake very greasy, and prevents its use for 
certain purposes. Moreover, it is not practicable on a commer- 
cial scale, owing to the cost involved in extracting this excess 
of oil. 

In making alizarine lakes it is important to use an alumina 
sulphate which is free from iron, as this latter ingredient has a 
great and deteriorating influence upon the colour of the resulting 
lake ; very small traces of iron are sufficient to give a brown 
hue to the lake. 

The methods of making lakes from alizarine, just described, 
are equally applicable to the preparation of lakes from the other 
alizarine dyestuffs. 

SCARLET LAKES may be made from a mixture of alizarine 
and alizarine orange. 

YELLOW LAKES can be made from alizarine yellow, 
galloflavine, gambine yellow, and flavazol, by any of the above 
processes, using alumina salts as precipitants. Generally a very 
satisfactory lake, of good colour, can be readily obtained from 
any of the dyestuffs named. 

ORANGE LAKE can be made from alizarine orange, or by 
using a mixture of alizarine and one of the yellows just named. 

MAROON or CLARET LAKES can be obtained from 
alizarine, by substituting acetate of chrome for acetate of 
alumina, in process No. 3 above. 

BLUE LAKES can be obtained from alizarine blue, alizarine 
cyanine, or chrome blue, by using chrome acetate as a precipitat- 
ing agent, and the process described in No. 3 of alizarine lakes. 

GREEN LAKES can be made from nitrosoresorcin and 



BROWN LAKES. 281 

gambine by using iron sulphate as the precipitating agent ; 
the lakes from gambine are rather brighter than those from 
the resorcin product Dark green lakes can also be obtained 
from coerulein by using chrome acetate as the precipitant. 

VIOLET LAKES can be made from chrome violet by using 
chrome acetate as the precipitant ; or from gallein and gallocy- 
anine, by using either chrome or alumina salts as precipitants. 

BROWN LAKES can be made from anthracene brown with 
chrome acetate as the precipitant. 

It has not been thought needful to give full details of the 
method for converting all the alizarine colours into lakes, as 
the process or processes, and the proportions of materials used 
are very similar to those which are used for making alizarine 
lakes, and these have been fully dealt with. Any intelligent 
colour-maker can easily apply the proportions and process for 
making an alizarine red to making an alizarine blue-lake. 

The great merit which distinguishes the lakes made from the 
alizarine group of dyestuffs is that of being permanent ; they 
resist a considerable amount of exposure to air and light without 
becoming faded or dull, they can, therefore, be used for decora- 
tive or artistic purposes where some degree of permanence is 
essential ; still, even in this property, they are by no means 
equal to such pigments as vermilion or yellow-ochre or chrome- 
yellow. 



282 



CHAPTER X. 
ASSAY AND ANALYSIS OP PIGMENTS. 

BESIDES the analysis required for ascertaining the chemical 
purity of a sample of a pigment, it is also necessary, with many 
of the pigments, to make an examination or assay for other 
properties such as colour or hue, brilliancy, colouring power, 
covering power or body, durability, fineness, and what may be 
called miscibility. In some pigments they are fairly constant in 
degree, while in others such as the chromes, ochres, umbers, 
blacks, &c. they are very variable, hence due precaution should 
be taken, both by purchasers and makers, that every lot of pig- 
ments is uniform, both in quality and intensity. 

1. COLOUR or HUE. The tint or shade of a pig- 
ment is a matter of the greatest importance. The terms, 
as used by colourists, are, however, rather confusing ; thus 
some persons consider tints to mean the standard colour 
mixed with white so as to obtain lighter colours ; shades 
they consider to be those produced by mixing the standard 
colour with black ; while other persons use these terms as 
if they were synonymous, and speak of the shade or tint of a 
colour without any reference to a standard. This, perhaps, is 
the custom more particularly in the dyeing trades. Under these 
circumstances it will be best to use a term which is free from 
any liability of confusion, viz., "hue." The hue of a colour may 
be denned as the optical effect produced on our colour-sense by 
a pigment. 

The hue of a pigment is a variable quantity ; in some cases, 
such as vermilion and antimony orange, where the chemical 
composition is a definite one, and does not vary with different 
makers and batches, the hue only varies within small limits. 
In other cases, such as the chrome-yellows, Brunswick greens, 
and many others, where the composition is liable to vary with 
different makers, and even with different batches of the same 
maker as also the ochres, umbers, and other natural pigments 
the hue varies very much. In such cases, the terms pale> 



COLOUR OR HUE. 283 

medium, dark, &c. are arbitrary distinctions which are by no 
means uniformly indicative of the same precise intensity of hue. 
Hence, in all such pigments, special examination is required in 
order to determine what the actual intensity is. 

The assaying of a pigment for hue can be done in two ways 
comparatively or absolutely; the former is the one usually 
adopted. 

Comparative Method of Assaying Pigments. In this 
method the hue of a sample is simply compared with that of 
a standard sample and the results stated in terms of this 
standard. For this purpose a sample of the best quality is 
selected, and a fair quantity of it is placed in a bottle. Colours 
which are acted on by light, such as the chrome-yellows, Bruns- 
wick-greens, and others, are placed in orange or amber coloured 
bottles to protect them from such action ; while white pigments, 
black pigments, ochres, umbers, and the permanent colours 
generally, are kept in ordinary bottles. These samples form 
the standards. 

The method of assaying the hue of a pigment by comparison 
with a standard sample is simple, but experience and a good 
eye for colour are essential requisites ; some persons can detect 
very small differences in the hues of pigments, while others are 
deficient in this respect, and consequently do not make good 
assayers of hue. The colour sense can be improved by cultivation. 

A sheet of black paper for pale colours, or of white paper for 
dark colours, is placed in front of a good window and in 
diffused light, this being the best for assaying hue. The paper 
must have a dead surface, as a bright one interferes with the 
observations too much. A small heap of the standard colour is 
placed on the paper by means of a palette knife, and beside it a 
similar heap of the colour to be compared ; then, by a gentle but 
steady downward pressure with the knife, the surfaces of the two 
heaps are flattened in such a way that a distinct line separates 
the two colours; if the knife is moved at all laterally, the two 
heaps of colours are partially amalgamated, and a perfect com- 
parison thereby precluded. The observer now carefully compares 
the two heaps of colour ; this should be done in several ways, 
viz., by looking down on them, by holding the paper on a level 
with the eyes and looking sideways at the colours, &c. ; by this 
means and with a little experience the differences in the hue of 
colours can be readily observed. 

There are two points in connection with the hue of a colour 
which can thus be compared with a standard colour : 1st, depth; 
2nd, tone. The two samples being compared may be equal in 



284 ASSAY AND ANALYSIS OF PIGMENTS. 

depth of hue or colour, yet different in tone. For instance, in 
two vermilionettes, the standard sample may have a bluish tone, 
while the sample compared with it may have a redder tone. 
Again, two samples of chrome-yellow when compared together 
in this way may show differences of tone ; one may be a greenish- 
yellow, while the other may have an orange tone. These differ- 
ences in tone are of quite as much importance as differences of 
depth of hue, as occasionally they will have an influence on the 
use of a pigment ; thus, an orange-toned chrome-yellow does not 
mix with Prussian blue to make greens as well as a greenish 
toned chrome-yellow does. 

Absolute Method of Assaying Hue. An absolute method 
of assaying pigments for hue may be founded on the use of 
an instrument devised by Captain Abney, named by him a 
"colour patch" apparatus. It is described in his book on 
Colour Measurement and Mixture. This method, while of some 
interest from a scientific point of view, is, however, scarcely one 
which will come into practical use in a colour shop, owing to 
its rather complex construction and to its requiring powers of 
experimenting beyond those of colour-makers generally. How- 
ever, a brief description of the process and apparatus may be use- 
ful. The colour patch apparatus consists, first, of a spectroscope, 
with which a spectrum of the light from an arc electric lamp can 
be formed on the screen of a camera \ by substituting a slide 
having a narrow slit in it for this screen and passing the light 
which comes through this slit through a lens, a patch of coloured 
light can be obtained on a screen placed behind the camera ; the 
colour of this patch will depend upon the position of the slit in 
relation to the spectrum which falls upon it and will necessarily 
be monochromatic. The arc electric lamp is preferred as the 
source of light, because it can be more depended upon than any 
other kind of light for uniformity in amount and quality, which 
feature is of importance where light measurements have to be 
made. 

Another part of the apparatus consists of an arrangement by 
which a disc of coloured card can be rotated ; the same apparatus 
also carries a larger pair of overlapping black and white discs, the 
amount of overlapping being capable of variation at will. This 
is placed so that the colour patch falls partly upon the coloured 
disc and partly upon the black and white discs ; these are rotated, 
and the slit of the colour patch apparatus moved along the spec- 
trum until a point is reached when the luminosity of the patch on 
the two discs is equal. Then a note is made of the position of the 
slit, as given on a scale attached to the colour patch apparatus, 



COLOURING POWER. 285 

and also of the relative quantity of black and white exposed in 
the black and white discs ; then another trial is made in the 
same way, only that the black and white discs are altered so that 
a different proportion of the two colours are exposed. It will 
now be found that equal luminosity of the colour patch on the 
discs occurs when the slit is in a different part of the spectrum 
to what it was before the two measurements were made. These 
measurements are repeated for various proportional exposures of 
the black and white discs. Then on a chart is drawn two sets 
of lines, viz., a horizontal set, to show the proportions of black 
and white ; and a vertical set, to show the position of the slit 
on the spectrum. Then by drawing a line through the points 
given by the various readings, a curve is obtained indicating the 
reflecting power of the particular pigment experimented with in 
each part of the spectrum. 

By carrying out this system with different pigments we are 
able to see how one compares with another. 

By a modification of the experiment the light reflected from a 
surface painted with a pigment can be compared with that which 
is reflected from a surface painted with a standard sample of the 
pigment. If instead of laying down the curve on a chart, it is 
drawn on a sector of a circle, so that the scale of the spectrum 
is measured off along a radius and the relative intensities on 
concentric circles, then a curve of somewhat different shape is 
obtained. If this be cut out it forms a colour template which, 
when revolved in front of the spectrum formed in the apparatus, 
cuts off just enough light that the remainder forms a colour 
patch of the same hue as the colour pigment ; by causing a patch 
of colour to be formed side by side with the colour patch, the 
relative hues may be accurately compared together. This second 
patch of colour is obtained by reflecting from a surface painted 
with the pigment. By making templates in this way and using 
the colour patch apparatus in the manner indicated, there is a 
certain method of comparing hues of pigments. 

BRILLIANCY or LUMINOSITY. This is an important 
feature of pigments, and one in which different makes of the 
same pigments are rather liable to vary. As with hue, brilliancy 
is assayed by comparison with a standard sample, and this can 
be done in precisely the same manner as described for hue. 

COLOURING POWER. Colouring power is that property 
of pigments which enables them to give colour to surfaces and 
to other pigments. As explained in another place, pigments 
possess two properties available for paint making, viz., colour 
and body, or covering power (see below). Some pigments are 



286 ASSAY AND ANALYSIS OF PIGMENTS. 

used almost solely on account of their colour, as, for instance, 
carmine, Prussian blue, ultramarine, vermilionettes ; hence with 
these strength of colour or colouring power is an important 
feature. Other pigments are used solely on account of their 
covering power, and then colour is immaterial. 

Colouring power is tested also by comparison with a standard 
sample. In principle it is done by ascertaining how much of 
another pigment it will colour to a given depth. 

Supposing it is a sample of vermilionette whose colouring 
power is to be determined, then 10 grms. of the sample are 
weighed out and mixed with 30 grms. of china clay; the mixing 
must be thoroughly done. 10 grms. of the standard sample are 
mixed in the same way with 30 grms. of the same sample of 
china clay. The two mixtures are now compared together for 
depth of colour as described above ; if the two samples are equal 
in colouring power, the depth of colour of the two mixtures will 
be the same ; if one is stronger than the other, then one of the 
mixtures will be darker than the other. Some idea of the 
relative strength of colouring power may be obtained by adding 
small and known weights of china clay to the darkest sample 
until the tint of the mixtures are equal to one another ; then 
the samples have a colouring power proportional to the amount 
of china clay used ; thus, if one sample took 30 grms. of china 
clay and the other sample 37*5 grms., then the relative colouring 
power is as 30 to 37 '5; or, if the strongest sample be taken at 
100, then the colouring power may be expressed in percentages 
thus, 37-5 :30 :: 100 : 80 ; the weakest colour has only 80 per 
cent, of the colouring power of the strongest. 

As the toning colour for all pigments except whites a good 
sample of china clay may be used ; gypsum also makes a good 
toning colour; barytes and white lead are a little too heavy. 
For whites a good animal black makes a good toning colour. 

When a large number of assays for colouring power have to be 
made a standard tint should be made by taking, say, 50 grammes 
of the standard sample, and mixing with about twice its weight of 
the toning colour; this tint may be used in subsequent tests, and 
will save some time in the preparation of a standard tint. It is 
important, however, that the same sample of toning colour be 
used to mix with the samples, whose colouring power is being 
tested, as has been used in making the standard tint. 

COVERING POWER or BODY. This is a most impor- 
tant property of pigment perhaps the most important, for those 
which possess it in the greatest degree are universally considered 
to be the best pigments. It may be denned to be the power of 



COVERING POWER OR BODY. 287 

covering over or hiding the surface of any body on which it may 
be spread when mixed into a paint. Some pigments, such as 
crimson lake, Prussian blue, and barytes, are deficient in this 
property ; others, such as white lead or the chromes, possess it 
in great degree. In the same pigment, however, the covering 
power is liable to vary to a greater or less extent. To some 
extent the covering power is dependent upon the condition of 
the pigment: if this is of an amorphous character, without any 
definite form of its own, and is opaque, it will, as a rule, be 
found to have good covering power or body ; on the other hand, 
if a pigment is of a transparent character, and is crystalline in 
its structure, then its covering power is liable to be small. 
Sometimes a pigment may be obtained in both conditions, 
according to the particular circumstances under which it is 
made; thus, lead chloride may be obtained as a white amorphous 
powder, or in small transparent crystals. In the former condition 
it may be used as a pigment, as it has some covering power; 
on the other hand, the crystalline variety is useless as a pigment, 
as it has no covering power at all. 

Unfortunately it is by no means an easy matter to devise a 
method of assaying the body of pigments; it cannot be expressed 
in absolute figures as can chemical composition, at the most it 
can only be assayed in a comparative manner as is colouring 
power. 

The best plan for assaying the covering power or body in pig- 
ments is to place 2 grammes of the standard sample and of the 
pigment to be compared with it on a black porcelain tile, and to 
add 3 grammes of oil to each; the oil and pigment are thoroughly 
incorporated by means of a palette knife, and then each is spread 
over the plate in a layer, making each layer of paint of as uniform 
a thickness as possible. That sample which, when thus made 
into a paint and spread over the tile, most completely obliterates 
the surface of the latter has the' most covering power. It is 
possible to obtain some idea of the comparative covering power 
of two samples by this method. To that sample which has the 
most body a small additional quantity of oil is added, and the 
body of the mixture again compared with the other sample ; if 
it is still the best more oil is added, and the process repeated 
until both samples appear to have the same covering power. 
Now, it may be assumed, without much error, that the covering 
power or body of the two samples is in proportion to the 
quantities of oil used to mix with them; that sample taking the 
most oil having the most body. Thus, of two samples of 
barytes one took 3 grammes of oil, and the other 3-25 grammes; 



288 ASSAY AND ANALYSIS OF PIGMENTS. 

taking the last as the standard or 100, the former had only 
3-25 : 3 : : 100 = 92-3, or 7-7 per cent, less body than the stan- 
dard sample. Or, to put it in another way, 3 : 3-25 : : 100 = 108 '3 ; 
that is, 100 Ibs. of the standard sample will cover as much 
surface as 108-3 Ibs. of the other, weaker sample. 

DURABILITY or PERMANENCE. Durability is one of 
the most important properties a pigment can possess, for upon it 
depends the fact whether it will ever come into extensive use 
as a pigment, especially for artists' use, where permanence is 
one of the most essential things a picture must possess. Until 
recently our knowledge regarding the permanence of pigments, 
or, what is the same thing, their power of resisting exposure to 
light and air, was empirical and unreliable; but recent researches 
on the subject have quite altered its character. 

Of those colours which have been shown to be permanent, the 
mineral pigments ochres, umbers, sienna, Vandyke brown, 
barytes, Chinese white, siennas, ultramarine, vermilion, Prussian 
blue, and some others are the most important, and can safely 
be used on work which is required to have great permanence. 

What is frequently sold under the names given to old pig- 
ments is, however, not always what it ought to be, being more 
or less adulterated with an inferior pigment ; it is, therefore, 
advisable to test a sample, not only for purity, but also for 
durability, as the adulterants are frequently anything but 
permanent. 

Of late years many new pigments have been placed on the 
market, which have been made from coal-tar colours; unfortun- 
ately, many of these, although of brilliant hue, are far from being 
permanent, and will not resist any lengthened exposure to light 
and air. It is desirable that a user of these pigments should 
make some experiments as to their durability. There is a 
great deal of difference in this respect among these coal-tar 
pigments. Some are as permanent as can be wished; others, 
again, are very fugitive. Then, again, the method of using has 
more influence on the durability of these pigments than it has 
on that of the older pigments ; for some which are rather 
fugitive, when used as water-colours, will resist a fair amount 
of exposure when used as oil-colours. 

Probably the simplest method (which is a very good one) of 
testing the durability of colours, is to provide a sheet of unglazed 
cardboard ; that known as Bristol board will do very well, It 
must have so slight an absorbent property that if any coat of 
paint is placed on the surface it will remain there, and not 
soak into the substance of the cardboard. This sheet of board 



DURABILITY OR PERMANENCE. 289 

is ruled into squares or rectangles, measuring about 3 x 2 or 
2x2 inches. 

A little of the colour to be tested is ground up with a little 
gum water into a smooth paste, and a portion of one of the 
ruled spaces on the cardboard painted with it. It is advisable 
to rule and prepare two sheets at the same time. The name of 
the colour can be written either underneath the patch of colour 
in the square, or in a corresponding position on the back of the 
card. It is also advisable to grind a little of the pigment with 
oil, so that the relative durability as a water-colour and as an 
oil-colour can be tested. 

One of the prepared cards is hung in a place where it is 
exposed to as much sunlight and air as possible, while the other 
card is placed in a drawer away from any such influence. After 
a week or two of exposure the cards can be compared to see if 
any changes have occurred; they can then be replaced in their 
respective positions, and from time to time are compared 
together. Any change which may have been brought about 
by the action of sunlight and air on the exposed card will 
be observable ', some colours will be changed in a few weeks' 
exposure, other colours require months of exposure to produce 
any effect. 

By placing a card painted in the manner described with dif- 
ferent pigments in a closed cupboard, in which is placed a vessel 
containing some ferrous sulphide and dilute sulphuric acid, the 
action of sulphuretted hydrogen on the colours can be tested ; 
if any are affected by this test it is certain that they will be 
similarly affected when exposed to the action of impure air. 

Testing pigments for durability is a very long operation, and 
it is no wonder that there have been few systematic researches 
on this subject. The most exhaustive and systematic experi- 
ments on the permanence of pigments which have ever been 
made are those made by Captain Abney and Dr. W. J. Russell, at 
the request of the Science and Art Department ; these extended 
over a period of two years, and the results were published in the 
form of a Blue Book, entitled " Report on the Action of Light 
on Water Colours." This report must be consulted for details 
as to the method of testing adopted, <kc. ; but the following will 
give some idea of the methods and results of these researches. 

The experiments were carried out as follows : A sheet of 
Whatman paper of good quality was covered with washes of the 
pigment to be tested in such a manner as to form a series of 
eight tints, varying from pale to dark. From this sheet, strips, 
8 inches long by 2 inches wide, and containing all the tints, 

19 



290 



ASSAY AND ANALYSIS OF PIGMENTS. 





. 

1 o 1 
52; 






1 




ll 



cfe c 



MIXABILITY. 291 

were cut, and two of them placed in a glass tube, 2 feet long 
and J inch in diameter, open at both ends, and bent over at 
the upper end in the form of a hook, in order to prevent the 
admission of dirt ; the tube was then hung in such a place as 
to receive as much sunlight as possible. It may be mentioned 
that one of the strips of paper was covered with oil-cloth so that 
it could not be acted on by light, although otherwise it was 
subjected to the same influences as the other strip. Similar 
tubes were filled with dry air, moist air, moist hydrogen, while 
one set had the air withdrawn so that the strips were exposed 
in a vacuum. The experiments lasted from May, 1886, to March, 
1888. The results of these experiments are given in the table 
opposite, which has been compiled from the tables given in 
the Report. 

Prussian blue fades when exposed to light, but on placing the 
faded colour in a dark place the colour comes back again. 

It is evident from the above experiments that moisture has 
a material influence on the durability of pigments ; colours which 
fade in moist air are permanent in dry air ; then, again, colours 
are more permanent in an atmosphere of moist hydrogen gas and 
in a vacuum than in air ; it is evident, therefore, that the three 
elements of destruction which cause colours to fade are light 
(which may be called the determining cause), oxygen, and water. 
But light alone has little effect. Hence it may be concluded 
that when a colour is kept under conditions where moisture and 
air can have little action it will be permanent ; so that it should 
always be kept under such conditions if possible. 

MIXABILITY. This is a term which the author has intro- 
duced in connection with pigments to express the power of pig- 
ments to mix more or less readily with oil or other vehicles or 
other pigments. It is a most important property, and much of 
the value of a body when used as a pigment depends on it; some 
pigments do not readily mix with oil, while some of the modern 
pigments made from coal-tar colours seem to have the property of 
retarding the drying of the oil, therefore they cannot be said to mix 
well with it. Then, again, in mixing pigments together to produce 
compound tints or shades, there are some pigments which can be 
mixed with all others without any ill efiect being observed ; on 
the other hand, some pigments can be mixed with a few others 
without any change occurring, but when mixed with others some 
action of a deteriorative character will take place. 

To test pigments for the property of mixability, the best 
method is to provide a sheet of cardboard of not too porous a 
character, and to rule this into squares of about 2 inches each 



292 ASSAY AND ANALYSIS OF PIGMENTS. 

way. A little of a pigment is rubbed with a small quantity of 
linseed oil in a white basin, during which operation its behaviour 
with the vehicle will be noticed ; it should be observed whether 
it mixes freely with oil or shows a tendency to separate out ; this 
latter effect may be due in most cases to the pigment not being 
thoroughly dry. A little of the mixed pigment is now rubbed in 
one of the squares of the card. Then prepare mixtures of the 
pigment with other pigments white lead, whiting, Prussian blue, 
emerald green, yellow chrome, vermilion, lamp-black rubbing 
the pigments together with a little gum-water ; these mixtures 
are rubbed on the card. When all the mixtures are ready the 
card is exposed to the air, and to diffused daylight only, for some 
time, say two or three weeks. After two or three weeks' 
exposure, the card may be examined and any effect of change of 
colour noted ; during the interval observations should be made 
as to the drying of the oil in the first square to see whether the 
pigment has any influence of any kind, either in retarding or in 
facilitating the drying; the former case will show that the pigment 
is not suitable to be used as an oil-colour, although it may be per- 
factly suitable as a water-colour. The other squares will show 
the action of one pigment on another; those which exhibit no 
alteration in shade or tint beyond, perhaps, a little fading will 
show the pigments which may be mixed together without any 
effect upon one another; while those which have altered will 
show the observer what mixtures to avoid. 

FINENESS. The quality of a pigment is a feature which is 
more or less dependent upon the size of its particles, the smaller 
these are (or, in other words, the finer the pigment has been 
ground or produced in the process of manufacture) the better will 
it be as a pigment; its body or covering power will be increased, 
its colouring power will also be improved and its tone brightened 
very considerably ; therefore, it may be laid down that the finer 
a pigment is the better it will be for use in painting. 

It is by no means easy to make a practical test for the fineness 
of a sample of pigment. By rubbing between the fingers it is 
possible to make a rough comparative examination, but no 
accurate results can be arrived at by this means; when the 
quality of two samples is very similar this rough test cannot be 
relied on. By spreading a little on a plain microscope slide and 
examining it through a powerful microscope, using, say, a ^-inch 
objective, some idea of the relative fineness of two samples may 
be obtained. Another method of testing which will give more 
reliable information and better comparative figures than the tests 
just noted is the following : Weigh out about 5 grammes into a 



FINENESS. 293 

mortar, and grind, without much rubbing action, into a smooth 
paste with water ; then transfer this to a tall cylindrical 
graduated measuring glass, and rinse out the mortar with water, 
so as to get the whole of the material into the glass ; fill this up 
with water to the top mark, and, putting in the stopper, shake 
well for a few minutes; then place on one side; the particles will 
gradually settle, and the time it takes for the water to become 
clear up to, say, the half-mark should be noted. If this be done 
with a number of samples a series of figures will be obtained 
which may be taken as showing the comparative fineness of the 
various samples, for the rate at which the material settles depends 
upon the fineness of its particles ; the finer these are the slower 
is the action, while the best samples are characterised by subsiding 
most slowly. Then this test will also show whether the sample 
is of uniform quality or whether it contains both coarse and fine 
particles ; in the former case the rate of deposition will be uni- 
form, while in the latter case the larger particles will settle out 
very rapidly, leaving the finer particles to subside more slowly. 
For example, the author tested three samples of china clay by 
this method. Sample A took 60 minutes to settle out; sample B 
44 minutes ; while the coarse part, forming the great bulk of 
sample C, settled out in 30 minutes, and the finer remainder in 
90 minutes. A and B were uniform in quality, but sample A 
was superior to B, being composed of the finer particles. Sample 
C was of mixed quality, containing both coarse and fine particles; 
the former settling out rapidly, the latter more slowly. C 
would not be so good as B for many purposes. 

This test cannot be used for examining the fineness of samples 
of different pigments : thus, for example, the comparative fineness 
of a sample of white lead and of barytes cannot be ascertained by 
its means ; only different samples of the same pigment can be 
compared together. This arises from the fact that the specific 
gravity of a body has a great influence on the speed with which 
its particles will fall when placed in water, and as there is a great 
difference in this respect between different pigments, it follows 
that a sample of white lead will settle out much quicker than a 
sample of china clay, although both samples may be equal in 
fineness of powder. 

The method of testing the fineness of the particles of a pigment 
here given is possibly not a perfectly accurate one ; but still 
some very useful information as to the quality of a pigment may 
be obtained by its means, and it does not need any elaborate 
apparatus to carry it out. _ 



UNIVERSITY 



294 



CHAPTER XL 
COLOUR AND PAINT MACHINERY. 

IN making colours or pigments, and in preparing them for use 
in painting, and in making paint, there are a good many mechani- 
cal operations which are common to all pigments and all paints. 
Upon the care with which the various mechanical operations, 
such as precipitating, drying, grinding, &c., are carried out, 
depends much of the quality of the pigment or paint, especially 
as regards its brilliance and covering power. The machinery 
for carrying out these various mechanical operations forms an 
important part of the outfit of a colour- or paint-shop. In the 
present chapter it is proposed to discuss the various machines 
which are required to carry out the various processes involved 
in making a pigment and its conversion into a paint. 

LEVIGATION. 

In the preparation of such natural pigments as the ochres, 
siennas, umbers, china clay, and barytes for use in paint-making, 
levigation plays an important part. These bodies as they are 
found in nature contain a good deal of gritty matter and other 
impurities, from which they must be freed before they are of 
use in paint-making ; there is no better process for this purpose 
than levigation. 

The principle of the process of levigation depends upon the 
fact that when fine particles of a comparatively light material 
mixed with coarser particles of the same material or with par- 
ticles of a heavier material are agitated with water and then 
allowed to stand, the coarser and heavier particles will fall first, 
while the lighter particles willform a layer on the top of the 
coarse particles, which can thus be separated from the fine par- 
ticles. A modification involving the same principle is where the 
mass of material is subjected ,to the sifting action of a current of 
water, the strength of which suffices to carry the fine particles 
only into a tank, where they are allowed to subside. China clay 



LEVIGATION. 



295 



is an example of the preparation of a pigment in this way (see 
p. 84). 

Should the raw material be made up of several distinct kinds 
of particles very fine, fine, medium, and coarse, as in the case 
of some ochres it is possible, by means of levigation, to separate 
them into their various constituents. By so arranging the 
current of water that it runs through a number of tanks with 
varying rates of speed, the coarse particles will be left in the first 
tank, the medium particles in the second tank, the fine particles 
in the third tank, and the very fine in the fourth or last tank. 

It will be seen that levigation, while effective, is a very cheap 
process ; for it only requires a cheap material, water, and the 
cheapest kind of colour plant, tanks, for carrying it out. 

The details of the plant required or used in levigating at any 
particular works depends upon many factors, such as the position 
of the works, whether situated in the centre of a town, in a wide 
valley, or on a hill side. The facilities for obtaining the requisite 
supply of water is also a factor in determining the arrangement 
of the plant. 

In Eig. 28 is shown in plan and elevation a plant suitable for 








Fig. 28. Levigating plant. 

levigating ochres, umbers, &c. It consists of 9 tanks, 8 of which 
are arranged in 2 series of 4, while the ninth is an odd tank. 
Another good arrangement would be one of 10 tanks, 9 arranged 
in 3 sets of 3, the tenth being an odd one. 

In the odd tank, A, the crude material is thoroughly mixed 
with water ; in Cornwall, Derbyshire, and a few other mining 



296 COLOUR AND PAINT MACHINERY. 

districts, this tank is known as the " huddle ; " in this the very 
heavy stuff remains while the current of water, which is con- 
tinually passing through, washes away the finer particles. From 
the huddle the water flows into the first settling tank 1 ; this 
being large, the current becomes retarded, and some of the 
material it contains settles out ; from tank No. 1 the water flows 
into tank No. 2 ; this is made, or should be made, rather larger 
than tank No. 1, so that the current being spread over a larger 
surface becomes slower, and, therefore, has less force, thus 
allowing the finer particles to settle out. From No. 2 the water 
flows to No. 3 tank, which is larger still ; and, finally, to No. 4 
tank, which is yet larger, so that very fine particles of pigment 
can settle out; when tank No. 1 is full the current from the 
buddle is diverted into the second series of tanks, while the 
colour or pigment in the tanks of the first series is settling out ; 
when this is completed the water in these tanks is run off, and 
the pigment dug out, when the tanks are ready to be filled again. 
By having a set of four settling tanks, four qualities of ochres, 
or siennas, or umbers, may be obtained. When the second series 
of tanks are full, the current is again sent through the first series. 
By having three series a more perfect system can be adopted ; 
the current of water is sent through the first series until these 
are full, then through the second series while the material in the 
first is settling out; when the second is full, the current is 
diverted to the third series ; by this time the colour in the first 
will have settled out, and can, as explained above, be collected ; 
when the first lot of tanks are emptied of their contents they 
are ready to be refilled by diverting the current from the third 
set of tanks. Thus the three operations of filling, settling, and 
emptying can go on concurrently in a complete manner. 

The tanks should be arranged, as shown in the drawing, one 
above the other, so that the water can run from one to the other ; 
and the last tank of the series should be of such a size that it 
will take a day to fill it. 

In ultramarine-making, where levigation forms an important 
part of the finishing process, the last tank is either made very 
large or a large number of small ones are provided, as the fine 
ultramarine takes a week to settle. 

When space is available it is a good plan to have a set of large 
storage tanks ; into these is thrown the wet pigment taken out 
of the settling tanks, and here it remains for some time ; a further 
settling takes place, and the pigment becomes drier ; this effects 
an economy of both time and fuel in the complete drying daring 
the final stage. This saving of fuel is a matter of some import- 



DRYING OF PIGMENTS. 297 

ance in dealing with such cheap natural pigments as china clay, 
umber, &c. 

The strength of the current of water is a matter that requires 
attention ; if too strong, it will carry over some of the coarse 
material from the buddle to the settling tanks, and will prevent 
the fine material from settling in the end tanks ; on the other 
hand, too gentle a current will not extract the whole of the 
valuable material from the crude stuff in the buddle ; this is a 
detail which the operator can easily arrange. 

If only small tanks are required they may be made of wood ; 
large tanks may be built of stone flags, or of brick, if flagstones 
of sufficient size are not available. If bricks are used the inside 
of the tank should have a smooth surface, so as to facilitate 
the ready removal of the colour which has settled out. In any 
case arrangements should be provided for running off the clear 
top liquor from the settled pigment ; this may be done by pro- 
viding in each tank a set of holes kept stopped by plugs, which 
are removed when it is desired to run the water away. Or the 
water may be syphoned off by means of syphons provided for 
that purpose. 

The amount of water required to levigate a pigment is a 
variable amount, depending on the nature of the sample of colour 
under treatment and on the plant used, so that no definite rules 
can be laid down. 

The size of the tanks can be varied to suit the required out- 
put of colour, and is a point which every colour-maker must settle 
for himself, remembering, first, the deposited colour will contain 
about half its weight of water, and will therefore be heavier than 
the dry material ; second, that the total volume of the tanks must 
be much larger than that of the material which settles out from 
them. Another point is to make the tanks sufficiently strong 
to bear the pressure of the water, &c., they contain, which is 
great ; thus a tank, 20 feet long x 5 feet broad and 4 feet deep 
will hold 20 x 5 x 4 = 400 cubic feet of water, or 400 x 62-35 Ibs. 
= 11 '13 tons, which is the pressure exerted by the water on the 
bottom of the tank. 

In some cases, before levigating, the material is ground, and 
in such cases the grinding is usually done under water ; for this 
purpose special mills are made, descriptions of which are given 
further on. 

DRYING OP PIGMENTS. 

After a colour has been prepared for use as a pigment by the 
process of levigation, as just described, or by that of precipitation, 



298 COLOUR AND PAINT MACHINERY. 

described below, and also by other processes, it is in a wet con- 
dition, probably containing from 25 to 50 per cent, of water, 
according to its nature. If required in what is known as the 
pulp state, in which condition it is used by paper-makers and 
stainers, no further treatment is necessary ; but, if required to be 
used in the preparation of paint, it is absolutely necessary that it 
be dried, otherwise it will not mix with the oil used in the 
manufacture of the paint. 

The drying of pigments is carried on in what are called 
" drying-stoves ;" these are usually nothing more than brick 
chambers with solid walls on three sides, and a door on the other, 
covered with a roof; round the bottom of three sides runs a 
horizontal flue belonging to a furnace which can be fed from the 
outside. The wet colours are usually placed in shallow, flat, 
earthenware pans, which are placed in piles one above another, 
and then left in until they are dry. This is by no means a satis- 
factory method, the piling of the pans, one above another, and 
the absence of any system of ventilation beyond accidental cracks, 
in the door and walls, tend to keep the atmosphere of the stove 
saturated with steam, and to check the drying operation. 

A better plan is shown in Fig. 29 ; it consists of a brick chamber 
built of any convenient size ; as before, the flue, F, of a furnace 
runs round the bottom ; the sides of the flue are built of brick, 
the top of flagstone, and the fireplace, E, is placed outside 
the chamber. Instead of such a flue, steam pipes may be used 
for heating it. Above the flue or steam pipes, is a staging, s, 
forming a false floor, on which is erected a framework, C, C, C, C, 
of iron or wood forming skeleton shelves on which the pans of 
wet colour are placed. These shelves support the pans a small 
distance apart from one another, and so allow free egress for the 
water- vapour which comes from the colour. A constant current 
of warm air, generated by a fan or air propeller, is continually 
flowing over the pans of colour and out through the ventilator, 
Y, in the roof of the stove, thereby carrying off the water-vapour 
as fast as it is given off from the wet colour. It should be borne 
in mind that the colour, just as it comes from the filters or 
presses, may contain from 25 to 50 per cent, of water ; if, by any 
means, this water is prevented from escaping from the colour, 
then the drying is retarded; or if it is prevented from readily 
escaping from the stove, it is liable to condense on the inside of 
the roof, and to fall down in drops on to the colour below. In 
some cases, e.g., chrome-yellows, these drops are apt to produce 
spots on, and discolouration of, the pigment which is being dried. 
The more freely the water- vapour can escape into the atmosphere 



DRYING OF PIGMENTS. 



291) 



the less chance there is of such mishaps occurring. D is a door 
for filling the stove, and G, G skylights. 




Fig. 29. Drying stove for pigments. 

In dealing with barytes and china clay, special forms of drying- 
stoves have been described. 



300 



COLOUR AND PAINT MACHINERY. 



Some pigments, like the two just mentioned, the oxide reds, 
burnt urnbers, burnt siennas, ultramarine, Guignet's green, are 
capable of standing a high temperature without being altered in 
shade; these may be dried in a stove heated to a high tempera- 
ture, in which case the drying is done quickly. On the other 
hand, certain colours, such as the chromes, Prussian blue, emerald 
green, &c., must be dried slowly ; for such colours the stove 
shown in Fig. 30 would be very useful. The two sides not 
represented in the drawing are of brick, and support the roof. 




Fig. 30. Drying stove for pigments. 

Stretching from side to side are a number of iron shelves just 
far enough apart to take an earthenware pan and leave a little 
space between it and the shelf above. These shelves do not 
stretch completely from back to front, but, as shown in the drawing, 
they are arranged to come alternately flush with the front and 
back, the side of the shelf nearest the front and back of each 
shelf being turned up to form a flange. The front and back of 
the stove are made of a number of iron plates, which form a 
series of doors to the shelves, the top of the plates being bent 
over to catch on the flange of the shelf above, as shown in the 



PREPARING PIGMENTS OR COLOURS BY PRECIPITATION. 301 

drawing ; it is not necessary that the doors should fit air-tight. 
A fan at the top of the stove creates a current of air through it, 
a chamber at the bottom is kept hot by steam pipes) or flue from 
a furnace ; through this chamber passes al] the air that is 
allowed to go into the stove ; this hot air passing over and under 
the colours dries them, and, being hot, absorbs and carries away 
the water vapour liberated from the wet colours. This stove is- 
effective and economical, and is so constructed that the pans of 
colour can be readily removed and the shelves quickly refilled. 

A drying stove has been constructed in the following manner. 
A cylindrical vessel was constructed of iron plates of any con- 
venient size. This was divided into three chambers by two 
perforated iron plates ; in the central chamber, which is the 
largest, is placed the material to be dried ; the bottom chamber 
is kept hot by means of steam pipes, and is provided with an 
opening to admit air. The upper chamber is fitted with an 
exhaust fan, so arranged as to draw the air out of the central 
chamber ; the perforations in the plate dividing the central from 
the top chamber are larger than those in the plate dividing the 
bottom from the central chamber, the consequence being that the 
air is drawn away from the central chamber faster than it enters 
from the bottom hot air chamber, so that a partial vacuum is 
created in the central chamber which is beneficial to effective 
drying. 

In any stove the colours are best placed in earthenware pans 
of about 12 to 16 inches in diameter, and 3 to 6 inches in depth ; 
smaller pans may be used, but it is not advisable to exceed the 
sizes just given. Pans made of galvanised iron have been used, 
but these are liable to rust and so lead to discolouration of the 
pigments dried in them ; enamelled iron pans, which can now be 
bought at a reasonable figure, are well worth a trial as being 
lighter and less liable to break than earthenware pans. 



PREPARING PIGMENTS OR COLOURS BY 
PRECIPITATION. 

Many colours the chrome-yellows, Prussian blues, Brunswick 
greens, lakes, &c. are prepared by a process of precipitation, the 
principle of which is that when two or more substances in the 
state of solution are mixed together a reaction sets in what the 
chemist calls double decomposition occurs and new products are 
formed; one of these being insoluble in the liquid used is thrown 
down or precipitated out of the solution, usually in the form of 



302 COLOUR AND PAJNT MACHINERY. 

a fine powder. Thus when to a solution of nitrate of lead, one of 
chromate of potash is added, a yellow powder falls down; this 
on examination is found to be chromate of lead, while the liquor 
contains nitrate of potash in solution; thus there has been an ex- 
change of constituents, the chromic acid has left the potash to 
form lead chromate, while the potash has combined with the 
nitric acid of the lead nitrate to form nitrate of potash ; the 
chromate of lead forms a precipitate because it is insoluble in 
water. 

Put into the form of a chemical equation this reaction is 
expressed as : 

Pb2N0 3 + K 2 Cr0 4 - PbCr0 4 + 2KN0 3 . 

Lead nitrate. Potassium Lead Potassium 

chromate. chromate. nitrate. 

Another example of precipitation met with in colour making 
is that of zinc sulphide, from solutions of zinc chloride and 
sodium sulphide, which is represented in the following 
equation : 

Zn C1 2 + Na 2 S = Zn S + 2 Na Cl. 

Zinc Sodium Zinc Sodium 

chloride. sulphide. sulphide. chloride. 

Here, again, there has been an interchange of constituents, 
and the zinc sulphide being insoluble is thrown down as a pre- 
cipitate. 

As precipitation is a chemical reaction it always takes place in 
fixed and definite proportions ; thus in the preparation of chrome- 
yellow it is found that 331 parts of lead nitrate interact with 
194 parts of potassium chromate, the result being that 323 parts 
of lead chromate are precipitated while 202 parts of potassium 
nitrate are left in solution; should the salts be mixed in any 
other proportion, then one or the other must be in excess, and 
this excess will be wasted ; thus, suppose 1 50 Ibs. of lead nitrate 
and 95 Ibs. of potassium chromate are used; the latter quantity 
is not sufficient to precipitate all the lead from solution; con- 
sequently, the excess, which is 7 Ibs., remains, and is practically 
wasted. The necessity of using equivalent proportions of the 
materials is a matter of importance as regards economy in making 
colours by precipitation. 

Every case of precipitation is a case of double decomposition, 
so that the main product is always associated with bye -products, 
which are sometimes worth recovering, or which may be utilised 
in other ways. Thus, in making chrome-yellow by the process 
mentioned above, potassium nitrate in solution is a bye-product ; 



PREPARING PIGMENTS OR COLOURS BY PRECIPITATION. 303 

in places where fuel is cheap it might pay to boil down this 
solution and recover the salt. Then again, by paying attention 
to the bye-products, and their probable use in other ways, it is 
possible to effect economies in the production of colours. Thus, 
supposing lead sulphate is to be made, this can be done by 
precipitating a solution of lead acetate, with either sodium sul- 
phate or sulphuric acid, as shown in the following equations : 



Pb2C 2 H 3 2 + Na 2 S0 4 = Pb S 4 + 

Lead Sodium Lead Sodium 

acetate. sulphate. sulphate. acetate. 

Pb2C 2 H 3 2 + H 2 S0 4 - PbS0 4 + 2HC 2 H 3 O 2 . 

Sulphuric Acetic 

acid. acid. 

In the first case, sodium acetate is formed, and although this 
has cost money, yet it must be thrown away, because it cannot 
be economically recovered by the colour-maker. In the second 
case, the acetic acid formed may be utilised in producing a 
fresh stock of acetate of lead from metallic lead; in this case 
there are no waste products, and the manufacture of the lead 
sulphate is conducted most economically. 

The character of the precipitate formed is modified in regard 
to its tint, its consistence, and in other ways by the conditions 
under which it is obtained. Thus, if barium sulphate be pre- 
cipitated from cold solutions, it falls down as a very fine, 
rather light powder, which is difficult to filter; but if thrown 
down from hot solutions, it is not so fine, and can be filtered 
more readily. Again, in making chromes, the conditions under 
which the operation is performed have a wonderful influence on 
the result; a difference in the lead salt influences the tint, the 
nitrate giving a finer product than the acetate. The temperature 
also modifies the result considerably. Thus, the chrome which 
falls down from cold solutions is much paler and more voluminous 
than that obtained from hot solutions. The acid or neutral state 
of the solution also has some influence, while the presence of 
such bodies as alum or sulphate of soda has a material influence. 

In making colours by precipitation, the following conditions 
affect the character of the resulting pigment: (1) Strength of 
solution. Generally, weak solutions yield finer and more volu- 
minous precipitates than strong ones. (2) Temperature. From 
cold solutions the product is usually finer and more volu- 
minous than from hot solutions. (3) Proportion between the inter- 
changing bodies. This also has some influence on the result. 
Thus, in making chromes it is preferable to keep the lead in 
excess; if a soluble Prussian blue is required, then the potassium 



304 COLOUR AND PAINT MACHINERY. 

ferrocyanide must be in excess. Other examples might be given 
illustrative of this point. (4) The order of mixing is important. 
If the lead salt, in making chromes, were added to the bichromate 
of potash, the pigment obtained would not be so fine as in 
adding the bichromate of potash to the lead salt. In making 
soluble Prussian blue it is important to add the iron salt to the 
potassium salt, not vice versd. In making emerald green, the 
acetic acid should be added to the copper before adding the 
arsenic preparation, if a good result is to be obtained. There is 
another feature in precipitation worth mentioning here. When 
a solution of a metallic salt is added to another solution con- 
taining two other salts, both capable of precipitating the first, it 
may happen that a kind of selective action may take place; the 
metallic salt will at first form a precipitate with one of the 
mixed salts only, and not until this action has ceased will it 
precipitate the other. An excellent example is the action of 
silver nitrate on a mixture of sodium chloride and potassium 
chromate; with the first it will give a white precipitate of silver 
chloride, with the second a dark red precipitate of silver 
chromate. When the two salts are mixed together, the silver 
nitrate will not precipitate the chromate until all the chloride 
has been thrown down, which point is shown by a change in 
colour of the precipitate from white to red. Another example is 
the precipitation of a mixture of sodium chromate and carbonate 
by zinc sulphate; in this case zinc carbonate is first thrown 
down. In precipitating a mixture of potassium bichromate and 
sulphuric acid with lead acetate, lead sulphate is thrown down 
before the lead chromate. This is a very interesting feature in 
precipitation, and is often taken advantage of by chemists for 
the separation of substances, one from the other; they know it 
as fractional precipitation. 

The plant required for making colours by precipitation 
is comparatively simple. There are required, 1st, vessels where- 
in to dissolve the various ingredients used ; and 2nd, vessels 
in which the ingredients are mixed together, precipitation 
vessels, which are preferably of wood, as those of earthenware 
would be too easily broken, and could not be made so large as 
required. The two classes of tanks or tubs should be kept 
distinct ; a dissolving tub should not be used for precipitating in. 
Then each colour that is made should have its own set of tubs. 
If a set of tubs were first used for making a chrome-yellow, and 
then for making a Prussian blue, the results in the second case 
would not be very satisfactory ; the rule of a colour shop should 
be that every distinct colour has its own set of tubs. The reason 



PREPARING PIGMENTS OR COLOURS BY PRECIPITATION. 305 

for such a course is that it is practically impossible to clean the 
tubs so as to avoid the liability of the remnants of one batch 
spoiling the next batch. 

Fig. 31 shows a good arrangement of plant for preparing pig- 
ments by precipitation ; a set of five tubs is shown ; two of 
these, CO, are placed on the floor, and used for the actual precipi- 
tation ; three, DDD, used for making solutions of the ingredients, 
and which may be smaller than C, are placed on a platform, P, 
above C. A steam pipe, S, with branch pipes, carry steam to all the 
tubs for the purpose of heating the contents, if that be necessary; 




Fig. 31. Plant for preparing pigments by precipitation. 

arrangements may be made for conveying water to these tubs. 
In the bottom of the tubs are plug holes or tap holes, which 
allow of the contents, when ready, flowing into C, through 
troughs. C has a number of holes, h, h, h, fitted with plugs, 
by the removal of which the supernatant liquors are easily 

20 



306 COLOUR AND PAINT MACHINERY. 

run off after the pigment has settled. Or the liquors may be 
siphoned off. Although a set of five are shown in the drawing 
as being an economical number, yet three only are, as a rule, 
required to be used at one time for preparing a pigment, but 
with five two batches of a pigment requiring the same materials 
may be more readily prepared. The size of these tubs must be 
proportioned to that of the quantity of pigment required to be 
turned out. A convenient size for the dissolving-tubs is 3 ft. 6 in. 
high, by 2 ft. 6 in. diameter; the capacity being about 110 gallons; 
the precipitating tub may be 3 ft. 6 in. high by 3 ft. 6 in. diameter, 
and will hold 320 gallons; batches of 7 to 10 cwts. of colour can 
easily be made in such tubs. 

The usual method of procedure in making pigments by precipi- 
tation is as follows. The materials, after being weighed out, are 
placed in the dissolving-tubs with the requisite quantity of water ; 
then, by means of the steam pipes, they are heated until complete 
solution has been effected. The liquors are now run into the 
precipitation tank, if they are to be used hot, or they may be 
allowed to cool before running into the precipitation tub. While 
running into the tub, it is desirable that the liquors be thoroughly 
mixed by stirring together. When all the liquors have been run 
into the precipitating tub, the mass is allowed to stand for the 
precipitate to settle ; when this has occurred, the clear liquor is 
run off, fresh clean water run in, the precipitate stirred up and 
again allowed to settle out, and the water run off; if necessary, 
this washing is repeated once or twice ; finally, the precipitate is 
allowed to settle, the clear top water run off as much as possible, 
the precipitate thrown on to a filter for the rest of the water to 
drain away, and the still wet precipitate placed in the drying 
stove to dry. 

In making some pigments, such as rose pink, the lake-pigments 
from the coal-tar colours, vermilionettes, &c., solid bodies such 
as barytes, whiting and orange-lead are added. This is best done 
by running one of the solutions into the precipitating tub, adding 
the barytes, <fcc., and stirring so as to get every particle of the 
solid thoroughly incorporated with the liquor; then the other 
solution is run in and the operation finished as described above. 
Unless c.are is taken to ensure the thorough mixing of the dry 
solid added with the liquors, the resulting pigment will have a 
speckled appearance. 

In preparing pigments from such natural materials as log- 
wood, cochineal, fustic, Brazil wood, &c., where the whole of the 
material is not dissolved, the actual colouring principle should be 
extracted by boiling in an apparatus so constructed as to permit 



FILTERING. 



307 



the colouring principle being extracted and removed by boiling 
from the insoluble particles of wood, <fec. Further details are 
unnecessary, as the extractors made are numerous and efficient. 
A simple form would be a boiler fitted with a perforated false 
bottom, on which the dye woods, &c., are thrown ; a similar per- 
forated plate is placed on the top of the mass, and the whole 
boiled with water ; when the extraction is considered to be 
finished, the liquor is run off below the false bottom, and through 
a filter into storage tanks, in which it is kept until required for 
use. 



FILTERING. 

Filtering, which follows precipitation, effects the separation of 
the pigment from the excess of water it contains. The simplest 
plan is to use a filter constructed of a sheet of calico loosely 
stretched over a skeleton framework of wood (see Fig. 32). The 
wet pigment from the precipitating tanks is thrown on to this 
sheet of felt ; the water drains through, while the pigment re- 
mains on the top of the sheet. 

On the whole, the process is an effectual one, and the filtration 




Fig. 32. Filter. 

is thorough ; the speed and completeness depends upon the 
character of the precipitate and the quality of the filtering cloth. 
If the precipitate is powdery and somewhat granular in structure, 
then the filtering goes on fairly rapidly, and the liquor that 
drains away is fairly clear. On the other hand, there are preci- 
pitates, such as Prussian blue and blanc fixe, that are very fine, 
rather slow in filtering, and from which it is difficult to obtain a 
clear liquor. 

in 

or THE 




FILTERING. 309 

The character of the filter cloth should be adjusted whenever 
possible to that of the pigment to be filtered; if of open 
texture it will filter rapidly, but for fine precipitates such 
as blanc fixe, Prussian blue, chrome-yellows, <kc., it is not 
suitable ; it can be used for coarse precipitates, like oxide of iron, 
Derby-red, lakes, <fec. Fine filter cloths filter slowly, but the 
filtration is thorough ; such cloths must necessarily be used for 
the fine pigments named above. 

The modern filter press, such as shown in Fig. 33, has not yet 
been generally adopted in colour shops, although its use would 
greatly facilitate the filtration of pigments. Its cost may exclude 
it from small colour works, but in large works, where quantities 
of pigments are turned out, its use will be found to save both 
time and labour. 

It consists essentially of a number of flat chambers, formed of 
iron frames covered with filtering felt, placed side by side on a 
suitable iron frame and pressed against one another by means of 
a screw or hydraulic press. The material to be filtered is 
pumped into the chambers, the liquor flows through the felt, 
while the pigment or solid matter is retained within the chamber. 
When it is considered that enough has been sent through the 
filter press, it is taken apart, the cakes of pigment removed from 
the chambers, and sent forward to the drying stove. 

The efficiency of these filter presses depends upon the work- 
manship given to the machine by the makers; every part must 
fit well, otherwise the filtration is imperfect. The quality of the 
filter cloths also has some influence. Generally, these presses 
require finer cloths than the simple filter described above, and, 
of course, fine precipitates will require finer filter cloths than 
coarse precipitates. These points can only be alluded to here in 
a general way; each colour maker must find out what quality of 
filter cloth will best suit the press he is using and the pigment 
he is filtering ; and in a similar way, the best conditions of 
pumping and the pressure used must be found out. 

Another method of filtering is by the vacuum filter press, one 
form of which is shown in Fig. 34. This consists of a hemi- 
spherical vessel of copper divided into two parts by a perforated 
plate. The top part is open ; the bottom part is closed ; but has 
two openings, one of which serves as the outlet for the liquor 
which runs into the lower part, while the other is connected 
with a vacuum pump. The mode of using is comparatively 
simple. The perforated plate is covered over with a sheet of 
filter cloth, the material to be filtered is thrown on to it and the 
vacuum pump set at work ; the liquor runs through, leaving the 



310 COLOUR AND PAINT MACHINERY. 

solid matter on the top of the filter. This method of working is 
very simple and efficient, quicker than the first method described, 
but not so quick as the filter press. Other forms of these vacuum 
niters have been made. One form consists of a conical vessel 
fitted on the top of a strong iron box ; between the two is a per- 




Fig. 34. Vacuum filter. 

forated plate or a sheet of wire gauze, on the top of which is 
placed a sheet of filter cloth; the material to be filtered is 
thrown into the cone, the air in the iron box is pumped out by a 
vacuum pump, the liquor is forced or rather pulled through, 
leaving the solid matter in the conical vessel. 



GRINDING. 

To fully develop the beauty of a pigment, and make it best 
adapted for conversion into paint, it is necessary to grind the 
materials very fine. Formerly this was done by hand on a 
slab of marble, with a muller. This is necessarily a slow and 
tedious process, only applicable to turning out small quantities 
of pigment or paint, and which is not admissible where large 
quantities are required, as in these days of ready-mixed paints. 
Grinding mills are, therefore, a necessary part of the plant of a 
modern colour-works. 

Grinding is peculiarly a mechanical operation, it is rarely 
that chemical action plays any part in it; the only examples are, 
perhaps, oxalate of iron yellow, Naples yellow, and mercury 
iodide scarlet, which are affected by iron and must be ground in 
stone mills. These pigments are only used by artists, and in 
small quantities. As a rule, the manufacturer of house paints 
need not trouble himself as to the material his mills are made of 



EDGE-RUNNER GRINDING MILLS. 311 

so long as they are efficient, although those with granite or stone 
grinding surfaces are to be preferred to those which have them 
of iron, as being least likely to affect the tints of the pigments 
ground in them. 

The grinding mills in use are constructed on one of three 
principles, and, therefore, may be divided into three groups : < 

1st. Edge-runner mills. 

2nd. Flat-stone mills. 

3rd. Roller mills. 

The first two kinds have been used for grinding purposes 
for centuries, so that nothing is known as to who invented 
them, or as to where they were first produced. Probably they 
were at first very simple in construction, and were afterwards 
modified so as to increase their grinding efficiency. The third 
group is of modern invention, having been introduced during 
the last forty years ; but even the name of its inventor is not 
definitely known. 

1st. Edge-runner Mills. This form of grinding mill has been 
in use for centuries for grinding corn, stones, and other materials. 
The essential principle of these mills is that of a circular stone 
or runner, set edgeways, and running in a circular basin-shaped 
trough or hopper and bed, on which the material to be ground 
is placed, and the stone rolling over it crushes it to powder. Many 
forms of this edge-runner mill are made, and it is also made in a 
great variety of sizes, suitable for the various materials usually 
ground in such mills. In some forms the trough and bed is made 
to revolve, while the runners are made to turn on fixed centres or 
axes by the friction of the bed on their edges ; in other forms the 
bed is fixed, and the runners rotate on a movable central shaft. 
In this case the runner has two motions one on its own axis, 
by friction with the bed, and another round the axis of the bed. 
In some mills the driving motion is applied from above, in others 
from below. Besides the arrangements for driving the runners, 
there is also a number of scrapers provided to keep the edge of 
the runners from becoming covered with a cake of the ground 
colour j other scrapers are fitted to the bed of the mill to keep 
the material under the runners, and prevent it from getting into 
the corners, where it would not be ground. Usually, also, the 
central shaft is provided with springs, so that should an excep- 
tionally hard piece of material pass under the runners the latter 
will tilt a little, and allow the piece to pass without doing any 
damage. 

Fig. 35 is a drawing of an edge-runner mill, such as is specially 
used for colour grinding. This is made in several sizes, but a 



312 



COLOUR AND PAINT MACHINERY. 



useful size is one having stones about 3 feet in diameter, and 
10 to 12 inches thick. Such a mill is 
capable of grinding from 1 to 1 J tons of 
colour per day, but the quantity will 
necessarily vary with the character of the 
material which is being ground. Some 
natural oxides are rather hard, and 1 ton 
may be taken as a fair day's work ; on the 
other hand, soft ochres may be ground at 
the rate of 2 tons a day. Much, too, 
depends upon the degree of fineness of the 
grinding; the finer it is the smaller will 
be the quantity turned out. 
In Fig. 36 is shown a section of Messrs. Follows & Bate's 
newest form of edge-runner mill. In this the bed is made of 
granite, 4 feet 8 inches in outside diameter ; the runners are also 
made of granite, and are 30 inches in diameter and 8 inches 
wide. The hopper is made of hard wood, the bearings of the 




Fig. 35. Edge-runner 
mill. 




Fig. 36. Edge-runner mill. 

runners of lignum vitse, and scrapers (not shown in the section) 
also of the same hard material ; thus the pigment or other 
material being ground does not come in contact with any metal ; 
therefore, it is not liable to be deleteriously affected. Further, 
the mill can be kept cleaner than is possible with a metal hopper 



PLAT-STONE GRINDING MILLS. 



313 



and bed, especially where the mill is only occasionally used. 
The mill is under-driven. 

In some respects under-driven edge runners are not so stable 
as over-driven mills, owing to the necessity of raising them above 
the floor level ; on the other hand, small mills are more con- 
venient to work with when driven, while for heavy grinding and 
large sizes of mills those which are over-driven present some 
advantages. Where in addition to a grinding action there must 
also be a mixing action, the edge runner mills will be found to 
possess some points of advantage over flat stone or roller mills. 

Edge-runner mills are very useful for the first coarse grinding 
of hard material, and for such natural pigments as the oxides, 
ochres, umbers, barytes, gypsum, <kc. They cannot compete 
with the flat-stone or roller mills for fine grinding. 

2nd. Flat-Stone Grinding Mills. Grinding mills con- 




Fig. 37. Flat-stone grinding mill. 

structed on the principle now to be mentioned have been known 
for a very long time, and have been made in several forms for 

f rinding all kinds of materials. As a rule, they are very efficient, 
n these flat-stone mills there are two grinding surfaces of a 
circular form, not necessarily (although usually) horizontal ; one 
of these (usually the lower one) is fixed, while the other is made 



314 COLOUR AND PAINT MACHINERY. 

to rotate on the top of the fixed surface ; the material to be 
ground is usually fed between the two surfaces at the centre, and 
the motion of the mill is such as to cause it to pass outwards to 
the edge ; in doing so it becomes ground. 

Fig. 37 represents a simple form. A is the top stone, which is 
usually about 1 foot thick and 2 feet in diameter. In the centre 
is fitted the driving shaft, E; the centre of the stone is hollowed 
out in a conical shape, forming a hopper, down which is fed the 
material to be ground ; usually a scraper, Sc, scrapes the material 
from the sides of the hopper, and causes it to pass down to the 
bottom. The fixed stone is represented at B, and is often a 
fraction larger than the revolving stone ; the ground material 
oozes out at the edge between the two surfaces, and is collected 
by a scraper, F, and passed down the shoot, S, into a receptacle 
for it placed below. Often a number of these mills are placed 
side by side on a bench, a shaft overhead supplying the necessary 
driving power; by suitable clutch-gearing the mills can be 
thrown into and out of gear as may be required. 

The quantity of material ground in such mills varies consider- 
ably, and is dependent on the kind of material and the degree of 
fineness of the grinding ; usually from ^ to 1 ton of material 
may be considered a fair day's work. 

The surfaces of the stones are usually corrugated so as to 
increase the cutting or grinding power of the stones ; these 
corrugations are arranged rather differently by various makers of 
these mills ; in some they radiate entirely from the centre, in 
others some radiate from the centre and some are more or less 




c 

Fig. 38. 

tangential to the circumference. Their shape has some influence 
on the grinding power of the stone mill ; the best form is that 
shown at A in Fig. 38, the stone moving in the direction of the 
arrow ; the back edge of the corrugation acts like a pair of 
shears on the material, and cuts it; the shape shown at B is 
bad, as such a stone has little grinding power. C is a shape 
commonly met with in mills with metal grinding surfaces. 



FLAT-STONE GRINDING MILLS. 



315 



For paste colours, it usually suffices to build a small trough 
round the edges of the stones, which serves to collect the ground 
colour as it oozes through from between the stones. 

Some makes of these flat-stone mills have the grinding surfaces 
constructed of hardened steel; or one surface of granite, the other 




Fig. 39. Levigating grinding mill. 

of steel ; or both grinding surfaces of granite. Modern makes 
drive the moving stone from underneath, in which case the top 
stone is a fixture, while the bottom is made to revolve. A 
variety of such a mill is shown in Fig. 39 ; Messrs. Rushton & 
Irving's levigating grinding mill, which shows very well the 
general form of such mills. When used to grind crude earthy 



316 COLOUR AND PAINT MACHINERY. 

pigments, like ochres, siennas, umbers, <fec., which are to be 
subjected to levigation, these mills are (like the one shown) 
cased in, the water aud crude material is sent in by the hopper 
on the top of the mill, while the water and ground material 
flows out through the spout below into the necessary settling 
tubs. 

To such mills screws are adapted to regulate the pressure 
between the grinding surfaces, and, therefore, the degree of 
fineness to which the materials are ground ; the greater the 
pressure, the finer the material ; on the other hand, the amount 
of material which can be ground in a given time is materially 
reduced. These flat-stone mills are usually very efficient, and 
some paint grinders prefer them to other forms. Certainly they, 
as a rule, take up less floor space than edge runners or roller 
mills, and a number can be readily and conveniently arranged 
on a stout bench along the wall of the grinding shop. 

Cone Mills. Another form of mills constructed on the prin- 




Fig. 40. Cone paint grinding mill. 

ciple stated above is that known as the cone paint mills, shown 
in Fig. 40, which is a drawing of one of Messrs. Follows & Bate's 
cone mills. These mills are always driven from underneath. 
Fig. 41 is the same mill shown in section, whereby the con- 
struction of the mill can be better seen. The top grinding- 
surface is formed by the bottom edge of a fixed hopper, 



CONE GRINDING MILLS. 



317 



into which the material to be ground is fed. The bottom 
grinding-surface is made of a conical shape, and fits closely 
against the edge of the hopper. The actual grinding surfaces 
are portions only of the hopper and the cone respectively, and 
these portions are usually corrugated so as to increase the grinding 
action, the corrugations starting from the centre and tapering 
towards the outer edges, to which, however, they do not extend. 
A screw arrangement under the mill serves to regulate the 
distance apart of the two surfaces, and consequently the fineness 
of grinding. The material is fed into the hopper, a revolving 
knife in which serves to keep the materials well mixed together; 




Fig. 41. Cone paint grinding mill. 

the ground material passes out from between the surfaces, and 
is scraped off by a suitable scraper ; in modern forms of this mill 
a trough is provided for the ground material to collect in ; this 
trough can be emptied from time to time as may be required. 

These cone mills are made in various sizes, from mills small 
enough to be driven by hand (Fig. 40) to those large enough 
to require power (Fig. 41). They are serviceable for all kinds of 



318 



COLOUR AND PAINT MACHINERY. 



materials, and particularly for wet paints. By being fitted with 
covers and covered spouts, they may be used for grinding 
enamel paints, varnish paints, and other like materials con- 
taining volatile liquids. 

The quantity of material these mills are capable of grinding 
varies with their size, but a mill having grinding surfaces 12 
inches in diameter will grind from 12 to 15 cwts. per day; but 
these mills can be had for grinding any quantity up to 40 cwts. 
per day. 

In Fig. 42 is shown a sectional view of Messrs. Follows <fe 
Bate's latest form of cone mill, which they call a disintegrator. 

This mill is adapted for 
grinding friable ma- 
terials which are offered 
to the mill in compara- 
tively large pieces. It 
will be seen that the 
grinding surfaces are 
very deeply serrated, 
especially where the ma- 
terial first comes in con- 
tact with them, while 
the extreme edges are 
not serrated, so that the 
materials are ground 
very finely. The ma- 
chine is strongly built, 
takes but a small power 
to drive, and does its 
work well ; every part 
is easily accessible for 
cleaning, while a cover 
prevents any of the ma- 
terial from flying about, 
This mill is best suited 
for grinding dry mater- 
ials, not for paste or wet 
colours. 

Messrs. Rushton, Irving & Co., of Liverpool, made an hori- 
zontal cone mill (shown in Fig. 43), in which the grinding 
surfaces are arranged vertically. Unlike other cone mills, the 
grinding surfaces are, in this mill, made of French burr stone, 
the fixed surface being made concave, while the revolving stone 
is made convex and ground, after being fixed on its spindle, to 




Fig. 42. Cone grinding mill. 



CONE GRINDING MILLS. 



319 



fit the concave surface. Both surfaces are furrowed to suit any 
special work for which they may be required. By means of a 
screw the pressure between the surfaces, and, therefore, the 




degree of fineness of grinding, can be regulated. This mill does 
its work well, takes up comparatively little floor space, and, for 



OF THE 
ttlVERSITT, 



320 



COLOUR AND PAINT MACHINERY. 



quality of work done, is much better than the edge runner mills. 
Owing to being placed direct on the floor and low in construc- 
tion, they are remarkably steady in running, and are convenient 
for feeding the rough material. 

3rd. Holler Grinding Mills. Roller grinding mills are only 
of comparatively modern introduction, but they are now in very 
extensive use for grinding all kinds of materials, and have dis- 




Fig. 44. Geyelin's roller grinding mill. 

placed the flat-stone mills to a very considerable extent. The 
first roller mill which was invented was patented in 1852 by an 
engineer named Jack, and consisted of a roller working in a 
curved horizontal surface, a form which presented many disad- 
vantages, and probably never came into extensive use. 

The form which the roller mill takes to-day was first patented 
in 1852 by J. K. Gfeyelin, whose mill is shown in Fig. 44. These 



ROLLER GRINDING MILLS. 321 

mills are, however, generally known as Clark's roller mills, from 
the name of the first maker, Mr. B. H. Clark. Geyelin's mill 
had three rollers horizontally placed in a suitable framing ; the 
rollers could be either conical or cylindrical ; the middle roller 
was the driving roller, and was connected with the other two by 
spur wheels arranged in such a manner that the speeds of re- 
volution of the three rollers were different. 

Screws for regulating the pressure between the rollers are 
provided, as also tension springs between the bearings, to allow 
a little play between the rollers in case any hard material should 
happen to accidentally get between them. A hopper between 
the first two rollers serves for the introduction of the material 
to be ground, while a scraper takes off the ground material from 
the last roller. Geyelin's mill includes many points of detail 
more fully worked out in the more modern forms of these roller 
mills. 




Fig. 45. Roller grinding mill. 

Fig. 45 shows a roller mill made by Messrs. Ritchie <fe Co., 
which has the three rollers arranged in an inclined plane, which 
it is obvious must be an improvement on the horizontal arrange- 
ment. A convenient size of such a mill has rollers measuring 
22 x 14 inches, the middle roller working at a speed of 55 revo- 
lutions per minute, the other rollers at different speeds ; it is 

21 



322 



COLOUR AND PAINT MACHINERY. 



capable of grinding from 3 to 4 tons of ordinary colours, or 
8 tons of white lead, in a working day of 10 hours. 

Fig. 46 is a drawing in section of Messrs. Follows & Bate's 
three-roller mill, with rollers 24 x 12 inches; all are driven from 




Fig. 46. Roller grinding mill. 

the central one with spur gearing, so that they revolve at dif- 
ferent speeds, the feed roller at 40, the middle roller at 80, and 
the delivery roller at 120 revolutions per minute; this difference 
of speeds much increases the efficiency of the grinding, and en- 
sures that the action of the mill shall be a grinding action, and 
not simply a crushing action, as would be the case if the rollers 
all revolved at the same speed. The central roller has a lateral 
motion imparted to it, whereby a further grinding effect takes 
place, and there is less risk of the rollers being worn into grooves, 
which would reduce their efficiency very much. Springs are 
placed in connection with the bearings, and by suitably-arranged 
screws the pressure of rollers on one another can be adjusted 
with nicety. A delivery scraper is provided, as also adjusting 
guides to regulate the amount of material supplied to the feed 
rollers. 



ROLLER GRINDING MILLS. 



323 



Fig. 47 shows the combination of the same grinding mill with 
two pug mills of the construction shown in Fig. 50. One pug 
mill only is shown in the drawing, but two are usually pro- 
vided with each mill. These are connected to the main driving 
shaft of the mill by chain gearing, so that a positive drive is 
obtained, and there is no slip or loss of power, as would be the 
case with a belt drive, and there is less noise than if a wheel 
gearing was adopted ; by suitably arranged clutches the pugs 
may be thrown in or out of gear as may be required. The con- 




Fig. 47. Combined roller and pug mill. 

tents of the pug mills are discharged on to the roller mill 
through sluices at the bottom. 

In Fig. 48 is shown a very powerful roller grinding mill made 
by Messrs. Follows & Bate. This drawing will also give a good 
idea of the general appearance of the roller mill and combined 
roller and pug mill. Figs. 46 and 47 show it in section only. 
The large mill really consists of two three-roller mills placed 
one above the other, so that the ground material from the upper- 
most mill, which gives the first grinding, passes direct to the 
lower mill, where it receives a second grinding. A pair of pug 



324 



COLOUR AND PAINT MACHINERY. 



mills is placed above the mills for mixing the materials together 
which are to be afterwards ground in the mills. The top mill 
and the pug mills are driven from the driving shaft by chain 
gear as shown in the drawing. 

In Fig. 49 is shown the form of roller mill as made by Messrs. 




Fig. 48. Combined double roller and pug mill. 

Brinjes & Goodwin. In this mill the rollers, three in number, 
are placed vertically above one another, two directly one above 
another, the third slightly on one side of the central line of the 
other two. The three rollers are not of equal size ; the two top 
rollers are about 10 to 12 inches in diameter, while the bottom 
roller is from 14 to 16 inches in diameter; they revolve at 
different speeds, the top or feed roller at 12 revolutions, the 



ROLLER GRINDING MILLS. 325 

middle roller at 46 revolutions, and the bottom roller at 110 
revolutions per minute ; this causes a considerable increase in 
the grinding power of the mill. Hand-screws serve to regulate 
the pressure between the rollers, while springs between the 
bearings enable the rollers to give when any hard material gets 
between them. The middle roller also has a small lateral motion 
in order to prevent the rollers from being grooved, and to increase 




Fig. 49. Three roller grinding mill. 

the grinding action. Scrapers are attached to each roller of this 
mill to keep the rollers clean and to ensure all material passing 
between the rollers. A delivery scraper is attached to the last 



326 COLOUR AND PAINT MACHINERY. 

roller to take off the ground material and deliver it into a suit- 
able receptacle. This mill will turn out from 75 to 100 cwts. 
of material per day, ground very fine ; it is an efficient mill, and 
invariably does its work in a satisfactory manner, while for very 
thin paint it is better than other forms of roller mills. 

In more recent makes of these mills the tendency is to increase 
the number of rollers ; for very fine grinding it is necessary to 
run the material twice through the mill to ensure it bein^ 
properly ground ; this repetition increases the labour attached to 
the process. To avoid this some makers have double mills with 
six rollers (see Fig. 48) in which the material is ground by the 
first three rollers and is then passed on to be re-ground by the 
second three rollers. Other makers arrange four or five rollers 
in one mill, so that an increased amount of grinding takes place; 
such mills are apt to get cumbrous and rather difficult to keep 
clean. 

Roller mills are only suitable for grinding colours mixed with 
oil ; they cannot be used for dry colours at all. The principle on 
which most of them work is that the colour, previously mixed 
with oil, is fed between the first and second rollers, where it gets 
a first grinding and, then, adhering to the middle roller, it is 
carried round, and again ground betweeft the middle and the last 
roller ; then, passing round the third roller it is scraped off by 
the delivery scraper and delivered into a receiver. The delivery 
scrapers are, in all the best makes of these mills, kept in contact 
with the third roller by balance weights which, on the whole, 
work better than if the pressure of the scraper on the roller was 
brought about by a wheel and screw arrangement. It is obvious 
that as the colour is in most forms carried between the second 
and third rollers by its power of adhesion to the second roller, 
that dry colours, having little or no such adhesive power, cannot 
be ground in these roller mills. 

Although a roller mill is capable of grinding a large quantity 
of material in a given time, it is generally considered that they 
do not grind colours so fine as does a well-built flat-stone mill. 



MIXING. 

An important part of the plant of a colour and paint shop is 
the mixing or, as they are often called, the pug mills, in which 
the dry colour is mixed with the oil, either to form the stiff paste 
in which so much of the painter's pigments is now sold, or to mix 
the pigment, oil, &c., into paint ready for use. 



MIXING MILLS. 



327 



The usual form of these pug mills is shown in Fig. 50, which 
represents one of Messrs. Follows & Bate's pug mills. It 
consists of a cylindrical casing 20 inches in diameter and 20 




Fig. 50. Pug or mixing mill. 

inches high; in the centre is a revolving shaft carrying arms 
which effectually mix the various ingredients together ; such a 
pug mill will mix 2 tons of white lead in an ordinary working 
day. It is customary to arrange a pair of these pug mills in con- 
nection with a roller grinding mill, as shown in Figs. 47 and 48, 
in which case arrangements are made for driving the pugs sepa- 
rately from the grinding mill, so that either one or both may be 
mixing at the same time, or one be mixing and the other deliver- 
ing its contents to the grinding mill through the openings in the 
bottom of the pug mill provided for that purpose. 

Messrs. Brinjes & Goodwin place the pugs, as also the stirrers 
or mixers, horizontally (see Figs. 51 and 5'J) ; the stirrers are of 
a form specially adapted for ensuring a thorough admixture of 
the various ingredients. They also arrange a pair of these to 
work in conjunction with their roller mill. 



328 



COLOUR AND PAINT MACHINERY. 




Figs. 51 and 52. Pug or mixing mill. 




Fig. 53. Mixing mill. 



MIXING MILLS. 



329 



Fig. 53 represents one of Messrs. Werner & Pfleiderer's uni- 
versal mixing mills. In it the trough containing the material to 
be mixed is placed horizontally ; there are two mixing blades of 
a peculiar form, which revolve in opposite directions, one within 
the other, so as to cause a thorough mixing of the materials. 
This peculiar construction of the mixing blades is shown in the 
illustration, which also represents the mill as being tilted for 
emptying ; the working position is with the trough horizontal. 
These mills are made in a great variety of sizes and forms, and 
are very effective, doing their work well and thoroughly ; they 
can be used either for dry or paste colours. 

A mixing mill of another pattern made by Messrs. Follows & 
Bate is shown in Fig. 54. This form is very good for mixing 




Fig. 54. Mixing mill. 

paint or dry colours together. The materials to be mixed are 
placed in a can or vessel, and this in turn is placed on an hori- 
zontal spur wheel whereby it is caused to revolve. A bunch of 



330 



COLOUR AND PAINT MACHINERY. 



stirring rods is made to revolve by suitable gearing so .as to mix 
the materials in the vessel. Arrangements are provided by 
means of which the stirrers can be lifted in or out of the vessel 
as may be required. This mill is an effective one, and capable of 
getting through a large amount of work. 

It is a good plan in mixing oil with the dry colour to pour the 
oil over the colour at least 24 hours before mixing and grinding, 
as the gradual absorption of the oil by the colour promotes and 
quickens their intimate mixture in the pug mill. 

The quantity of oil required to grind colours into the stiff paste 
in which they are now so largely sold varies very considerably 
with different pigments ; some only require a comparatively small 
quantity of oil, others a relatively large quantity. Even with 
different samples of the same colour the proportion will vary a 
little. Different colour makers, too, use different proportions of 
oil and dry colour in grinding. The following table will give 
some idea of the proportions usually adopted, which are essen- 
tially the same both for raw and for boiled linseed oil : 



White lead, 

Zinc white, . , 

Barytes, 

Putty, 

Black, 

Brunswick green, 

Red oxides, 

Brunswick blue, 

Oxford ochre, 

Burnt Turkey umber, 

English umber, . 

Vandyke brown, 

Siennas, 

Black in turps. , 



cent, of oil. 



7 

18 
27 
11 
10 
11 
164 
29 
20 
40 
374 
55 



These figures are based on practical working, but 



turpentine. 



as mentioned 



above, are liable to vary a little from time to time 

Finally, the mixing and grinding of paints and colours cannot 
be too well done, as these operations materially influence their 
brilliance and covering power. 



331 



CHAPTER XII. 
PAINT VEHICLES. 

THE most convenient way of applying pigments to the surfaces 
of bodies is by mixing them with certain fluid bodies called 
" vehicles," which act both as carriers and as fixers. Vehicles 
may consist either of a single fluid, of a mixture of liquids, or 
even of a liquid containing a solid body in solution, which may 
act as the real fixing agent, the fluid simply acting as a solvent 
for this and a convenient medium wherewith to mix the pigment 
with it for use as a paint. The usual vehicles are certain oils, 
turpentine, shale spirits, benzoline or petroleum spirit, benzol, 
coal-tar naphtha, methylated spirit or alcohol and water. Each 
of these is used in certain classes of paints and varnishes. 

PAINT OILS. The oils are a numerous group of bodies 
derived from both animal and vegetable sources. The char- 
acteristic features of oils are that they are lighter than and 
insoluble in water, are rather viscid fluids, are greasy to the feel, 
and impart a permanent greasy stain to paper; they are only 
partially soluble in alcohol, but are freely soluble in ether, 
petroleum spirit, and turpentine, and some other solvents of a 
similar character. Boiled with caustic soda (sodium hydroxide), 
they are decomposed, yielding soap and glycerine. All the oils 
are not suitable for use as paint oils; they may be divided into 
two principal groups with some sub-groups: 1st, Those oils 
which on exposure to the air do not change, or, at most, become 
slightly more viscid. 2nd, Those oils which on exposure to air 
gradually become hard and dry; these oils are called the drying 
oils, and are used as paint oils on account of their possessing this 
important property. The first group of oils are known as the 
non-drying oils ; they are quite useless as paint oils, and are never 
used for that purpose. 

The oils belong, chemically, to that group of bodies known as 
salts, which may be defined as compounds containing two 
radicles, one of which is of acid origin, the other of basic origin. 

In the case of oils the latter is always the body known as 



332 



PAINT VEHICLES. 



glyceryl (0 3 H 5 ), which when combined with hydroxyl (HO) 
forms the well-known compound, glycerine, C 3 H 5 (H O) 3 ; hence 
oils are frequently known as glycerides because on saponification 
they yield glycerine. In the oils this glyceryl is united with 
various acids ; for, while there is only one base present in any oil, 
there is rarely less than two acids present, and often there are 
many more. These acids are known as the fatty acids, and form 
a rather numerous group, or rather several groups of bodies. 
Some of these, such as oleic, stearic, palmitic, linoleic, are found 
present to a greater or less extent in all oils; others, such as 
arachidic, ricinoleic, valeric, &c., are only found in small 
quantities, and often only in certain oils of which they are the 
characteristic constituent, such as ricinoleic acid in castor oil, 
arachidic acid in ground nut oil, valeric acid in fish oils, rapic 
acid in rape oil, linoleic in linseed oil. 

The fatty acids may be divided into three groups, which, 
from the most prominent acid they contain, may be named 1st, 
the stearic acids; 2nd, the oleic acids; 3rd, the linoleic acids. 

The first group is a very numerous and important series of 
acids, and is often called, from the most important member of the 
series, the acetic acid group. Many of these bodies, such as acetic 
acid and stearic acid, are used on a large scale in various 
industrial operations ; others are of importance as occurring in 
products which are of great industrial value. The following lists 
comprise all the known members of these series : 

1. STEARIC SERIES OF FAT ACIDS. 



Name. 


Formula. 


Name. 


Formula. 


Formic, 








HCH0 2 


Pentadecatoic 






H Ci5 H 2 9 2 


Acetic, 








H C 2 H 3 2 


Palmitic, 








HCi 6 H 31 2 


Butyric, 








H C 4 H 7 2 


Margaric, 








H C 17 H 33 2 


Caproic, 








HC 6 H H 2 


Stearic, 








HC 18 H 35 2 


(Enanthyli 


j 






H C 7 H 13 2 


Arachidic, 








H C 2 o H 3 9 2 


Caprylic, 








H Cg HIS 2 


Medullic, 








H C 2 i H 4 i O 2 


Pelargonic 








H Cg HI 7 O 2 


Behenic, 








HC 22 H 43 2 


Capric, 








H CIQ Hjg 2 


Lignoceric 








H C 24 H 47 O 2 


Cocinic, 








H GH H 2 i O 2 


Geoceric, 








H C 2 e HSI Oj 


Laurie, 








H Ci 2 H 23 O 2 


Cerotic, 








H C 27 HSJJ O 2 


Tridecatoic, 






HC 13 H 25 2 


Melissic, 








H C 3 o HSQ Og 


Myristic, . 






H C 14 H 27 2 







Formic and acetic acids are liquids having a powerful acid 
odour, are soluble in water, and can be distilled without change. 



PAINT OILS. 



335 



The next few members of the series are liquids more or less 
soluble in water, and can be distilled without change; they have 
a slight odour of rancid fat, and are known as the soluble fat 
acids, being present in such fats as butter, cocoa nut oil, and palm 
nut oil, and are occasionally found present in small quantities in 
fish oils. The higher members of the series, or from capric acid 
upwards, are solids; they are insoluble in water, and cannot, as 
a rule, be distilled without being decomposed. 

The fat acids are soluble in alcohol, ether, turpentine and 
similar solvents; they are monobasic acids combining with one 
equivalent of potassium hydroxide (caustic potash) or sodium 
hydroxide (caustic soda) to form soaps, which are more or less 
soluble in water, the salts of the lower fatty acids being freely 
soluble, while those of the higher acids are rather difficultly 
soluble, the solubility decreasing with the complexity of the fatty 
acids. 

2. OLEIC SERIES OF FATTY ACIDS. 



Name. 


Formula. 


Name. 


Formula. 


Acrylic, 


H C 3 H 3 2 


Physetoleic, ) 




Crotonic, . 

Angelic, 


H C 4 H 5 2 
H C 5 H 7 2 


Hypogaeic, [ 
Gaeidic, . ) 


H CIG H 2 9 2 


Pyroterebic, 
Damaluric, 


H Cg Hg O 2 

.H (uf Un O 2 


Oleic, . . \ 
Elaidic, . / 


H Cjs H33 2 


Damolic, . 


H Ci3 H 2 3 2 


Doeglic, 


H Cj9 H35 2 


Moringic, . ) 
Cimicic, . \ 


H C 15 H 27 2 


Brassic, . ) 
Erucic, . . j 


H C 22 H 41 2 



These acids are very characteristic of fats and oils ; oleic is by 
far the commonest of all fat acids, as when combined with 
glyceryl it forms olein, the fluid constituent of almost all oils. 
The lower members are more or less soluble in water and vola- 
tile by heat without decomposition; the higher members are 
insoluble, and are decomposed by heat. 

3. LINOLEIC SERIES OF FAT ACIDS. 



Name. 


Formula. 


Linoleic, 


HC, 


6 H27 2 


Homolinoleic, .... 


Hy-1 
^1 


8 H 31 2 


Ricinoleic, . . . . . 


Hd 


8 H 33 3 



334 PAINT VEHICLES. 

Linoleic and homolinoleic acids are characteristic of linseed and 
other drying oils, while ricinoleic acid, which has properties 
very different from other acids, is found only in castor oil. 

Both the oleic and linoleic series of acids are monobasic, 
like the stearic series, and combine with potash and soda to 
form soaps which are rather more soluble in water than the 
soaps made from the stearic acids. 

Glycerine, the sweet spirit of oils, is a water-white, very 
viscid liquid, quite odourless, but possessing a sweet, though 
metallic, sort of taste. It is heavy, having, when pure, a specific 
gravity of 1'270. When heated it volatilises with difficulty, 
being slightly decomposed during the operation. It will only 
burn when heated, and then with a smoky flame having a small 
amount of luminosity. 

It has a great affinity for water, with which it mixes in all 
proportions, and which it absorbs from the atmosphere in no 
small proportions, being strongly hygroscopic. On this account 
glycerine gradually becomes weaker when exposed to the air. 
It is soluble in alcohol. 

Glycerine is a compound of the basic organic radicle glyceryl 
(C 3 H 5 ), with three equivalents of hydroxyl, O H. and has the 

(OH 
formula C 3 H 5 < H It possesses alcoholic properties, and is 

(OH 

capable of combining with acids ; with monobasic acids it 
requires three equivalents to form saturated salts, and hence 
is capable of forming three different compounds with such acids ; 
thus, with oleic acid it forms 

OH (OH 

Monolein, C 3 H 5 O H Diolein, C 3 H 5 C 18 H 33 2 



!C 
Ci 8 H 33 2 
G 



in which one, two, or three equivalents of hydroxyl are replaced 
by one, two, or three equivalents of oleic acid. These compounds 
can be formed by the direct union of oleic acid and glycerine, 
and it is of interest to note that the triolein so made is indis- 
tinguishable from the olein which is present in oils. 

When oils are boiled with solutions of caustic soda or potash 
they are decomposed. A compound of alkali and fat acid is 
formed while glycerine is liberated. This reaction is shown in 
the following equation : 



THE DRYING OILS. 335 

(C 16 H 27 2 (HO 

C 3 H 5 \ C 16 H 27 O 2 + 3 Na H = 3 Na C ]6 H 27 2 + C 3 H 5 I H O 

(C 16 H 27 2 (HO 

Linolein. Sodium Sodium Glycerine. 

Hydroxide. Linoleate. 

This reaction is known as saponification, because the alkaline 
compound obtained forms the familiar article, soap. 

As stated above, the oils are divisible into two groups non- 
drying and drying oils. The former group is by far the larger 
of the two, but the oils in it are of no use to the painter. The 
latter group, which is the one which will be dealt with in this 
chapter, is small in number, but it contains oils of great im- 
portance to the painter. 

THE DRYING OILS. 

There are but few oils, compared with the great number 
known, that can be used for painting on account of their pos- 
sessing the essential property of becoming hard or drying when 
exposed to the air. The best drying oils are obtained from 
vegetable sources, although one or two are obtained from animal 
sources. The following list includes all that can be included 
within this group : 

Linseed oil. Hempseed oil. 

Poppy seed oil. Tobacco seed oil. 

Weld seed oil. Walnut oil. 

Firseed oil. Japanese wood oil. 
Menhaden oil. 

A few other oils, such as niger seed oil, cress seed oil, grape 
seed oil, cotton seed oil, possess weak drying properties, but 
these are of much too weak a character to permit of their being 
used as paint oils. Rosin oil is also offered as a paint oil, but 
it is not a good drying oil ; it will be dealt with in detail 
further on. 

The most important of this group of oils is linseed oil, which 
is the painter's oil par excellence ; the others are only used in 
painting on a comparatively small scale, and mostly by artists, 
not because they are any better drying oils than linseed, but 
because they have a paler colour, and, therefore, do not affect 
the tints of the colours quite so much a matter of some im- 
portance when delicate tints have to be painted. For house- 
decorating purposes no other oil is used, because no other is so 
cheap or abundant. 




tTNIVERSITT 



336 PAINT VEHICLES. 

LINSEED OIL. This oil is obtained from linseed i.e., the 
seeds of the flax plant, Linum usitatissimum which is culti- 
vated both for its seed and for its fibre, which latter is spun and 
woven into linen. Ireland, England, Holland, Germany, Russia, 
America, Canada, and India are noted for the cultivation of the 
flax plant, which will grow anywhere where the climate is not 
too hot, but a cold or temperate climate suits the plant best. 

Linseed is a small seed of a flat oval shape, somewhat pointed 
at one end ; it is lustrous, and of a pale brown colour. It varies 
a little in shape and colour, according to the locality in which 
it is grown, and an expert can tell by inspection without much 
error from whence a particular parcel of seed has come. It is 
exported in large quantities from Riga, Libau, Taganrog, and 
other Russian ports, and from Calcutta in India. Smaller quan- 
tities come from other places, but these occupy only a minor 
position compared with the seed coming from the places named 
above. Three qualities of linseed are recognised in the trade 
Baltic seed, which comes from Riga and other ports on the 
Baltic coast of Russia, and is the seed of flax grown in the north 
of Russia; Black Sea seed, coming from Libau, Odessa, and 
other ports on the Black Sea, and which is the seed of flax 
grown in Southern Russia ; and East India seed, which is 
exported from Calcutta. The Baltic seed yields the best and 
most valuable oil, that from Black Sea seed is next in quality, 
while East India seed gives oil of inferior quality. 

The oil obtained from seed grown in America, Canada, and 
other places, is mostly used locally, and very little finds its way 
to the English market. The seed imported is rarely free from 
other seeds such as those of hemp and rape. 

Extraction of Linseed Oil. The oil is obtained from 
linseed and other seeds by a process of pressing; but, before 
being pressed, the seeds undergo some preliminary treatment, 
with the object of facilitating the process of oil extraction. 
Naturally the process of extracting oil has undergone many 
changes during the last fifty years, and for the purpose of this 
work, it will be sufficient if the modern systems only are 
described in outline; for further details, the companion work on 
Lubricating Oils in this series of text books may be consulted. 

Two systems of oil extraction are used viz., the English and 
the Anglo-American; these do not differ very much from one 
another. 

The English system includes five operations: 1st, crushing; 
2nd, grinding; 3rd, heating; 4th, pressing; 5th, refining. 

1st, Crushing. Prior to this, however, the seed is sifted 



EXTRACTION OF LINSEED OIL. 



337 



through sieves, with the object of separating out as much dirt 
and foreign seed as possible; this is a preparatory operation, 
common and essential to all systems of oil pressing, as the 
presence of much oil from other seeds is apt to spoil the linseed 
oil. As the majority of the foreign seeds found in the crude 
linseed are rather smaller than linseed itself, nearly all are 
separable by using sieves of a certain gauge of mesh. 




Fig. 55. Oil seed crushing mill. 

The sifted seed is now passed through a crushing machine, 
which consists of a pair of heavy wheels placed horizontally 
in a strong frame (Fig. 55). One of these wheels is 4 feet in 
diameter, and is driven by the one to which power is applied ; 
the other wheel, one foot in diameter, is generally driven by 
friction from the larger wheel, but in some makes of the mills 
by gearing ; by means of a system of screws and springs in 

22 



338 PAINT VEHICLES. 

connection with the bearings of the small wheel, a regulated 
pressure can be brought to bear on any seed which passes 
between the two wheels. The large wheel is driven at about 
fifty-six revolutions per minute. A hopper placed above the 
wheels supplies the seed in a regular manner, which is crushed 
in passing between the wheels, and falls into the receptacle 




Fig. 56. Oil seed crushing mill. 

placed underneath the machine for the purpose of collecting the 
crushed seed. Attached to each wheel is a scraper for keeping 
the surface of the wheel free from crushed seed. 

2nd, Grinding. From the crushing machine the seed passes to 
the grinding mills, which are of the edge-runner type (Fig. 56), 
and consists of a pair of large stones, about 7 feet in diameter, 



EXTRACTION OF LINSEED OIL. 339 

and 16 inches thick, revolving in a rather flat-shaped pan. 
The stones weigh about 7 tons, and have a speed of about 17 
revolutions per minute. In this mill, the seed is ground for 
about 20 minutes, a little water being usually added. After it 
has passed through the edge-runner mill, the seed is in the form 
of a fine pasty mass. 

3rd, Heating. After being ground under the edge-runners, the 
seed is placed in a kettle, a cylindrical vessel made of copper, 
and jacketted, so that the inner portion and the seed it contains 
can be heated; agitators worked by suitable means are also 
provided to ensure that every part of the seed is heated. In 
this kettle the seed is kept for 20 minutes at a temperature 
of about 150 to 160 F. This heating of the seed serves two 
useful objects; it liquefies the oil, and so causes it to flow more 
freely from the seed while being pressed; and it causes the 
coagulation of the albuminous and mucilaginous matters 
contained in the seed, and so prevents these from flowing out of 
the seed during the pressing. 

1th, Pressing. The hot seed from the kettle is placed in 
woollen bags, large enough to hold about 8 Ibs. of seed; these 
are first placed in horse-hair cloths, generally known as "hairs"; 
and then between the press-plates of an hydraulic press, where 
they are subjected to a pressure of about 740 Ibs. on the square 
inch for 20 minutes, followed by a pressure of about 2 tons for 7 
minutes; during the whole period, oil flows out from the seed 
and into suitable vessels which have been placed ready to 
receive it. 

5th, Refining. The oil as it flows from the press is far from 
being pure and bright; to ensure these qualities it undergoes a 
process of refining, the details of which vary in different works. 
First, it is usually placed in large tanks and heated by means of 
a steam coil to about 160 or 170 F.; this makes the oil more 
fluid, and causes the coagulation of any albumen which may 
have passed out of the seed; it is allowed to stand for some time, 
until all the solid impurities have settled, leaving the oil fairly 
bright; occasionally it is sold in this condition, but generally it 
is subjected to a further treatment, which consists in mixing it 
with 3 per cent of sulphuric acid, previously mixed with an 
equal quantity of water; the two bodies are thoroughly mixed 
together and then allowed to stand for 24 hours, when the acid 
will have settled to the bottom, leaving the oil at the top; the 
acid layer is run off, and the oil washed free from any traces of 
acid by means of warm water; it is then ready for use. The 
sulphuric acid has the property of acting upon any albuminous 



340 



PAINT VEHICLES. 



or mucilaginous matter contained in the oil, charring and 
dissolving it, thus carrying it out of the oil, leaving the latter 
in a purer condition, and, by removal of bodies which are prone 
to decomposition, rendering the oil less liable to grow rancid ; 
while in other respects its properties are materially improved. 
The oil as thus made is sold as "Raw Linseed Oil." 

The Anglo-American system is comparatively of recent intro- 
duction, but already it is most extensively adopted and will in 
all large oil mills be used exclusively. Like the English system 
it includes five operations, some of which resemble those of the 
system just described, while others are different: 1st, crushing ; 
2nd, heating ; 3rd, moulding ; 4th, pressing ; and 5th, refining. 

1st, Crushing. This operation is done by means of a crushing 




Fig. 57. Oil seed crushing rolls. 

mill, shown in Fig. 57, which consists of a number (usually five) 
of chilled iron rolls placed one above another in a strong iron 
frame ; the size of the rolls varies according to the required capa- 
city of the machine, but a usual size is two feet six inches long, 
and one foot in diameter. The seed is fed by means of a hopper 
between the first and second rolls, and passing between the nip 
of these receives a first crushing ; then, passing round the second 
roll and between the second and third rolls, it receives a 
second crushing as it passes through the nip of these rolls ; the 



EXTRACTION OP LINSEED OIL. 



341 



crushing weight now exerted on it is greater than at first, the 
weight of the first roll in the first crushing being augmented 
by that of the second roll in the second crushing. As some 
of the crushed seed may adhere in the form of a cake on the 
surface of the rolls, scrapers are provided to scrape any such 
seed off the rolls and guide it through the various nips. In the 
same way as it has passed through the first, second, and third 
rolls, the seed passes through between the nips of the other 
rolls, being further crushed in so doing, the weight to which it 
is subjected increasing as it passes through the various nips ; in 
a roll machine having rolls of the size given above, each roll will 
weigh about li tons, so that if there are five rolls, the final 
pressure on the seed will be from 5J to 6 tons. The seed is 
crushed by such a set of rolls in a very perfect manner, much 
better than in the older system of a pair of rolls and an edge- 
runner grinding mill, and less labour is required. 

2nd, Seating. From the 
crushing mills the seed 
passes to the kettle, the 
construction of which is 
shown in Fig. 58 ; here the 
seed is heated to a tem- 
perature of 170 F. for 20 
minutes. Steam is sent 
into the interior of the 
kettle to moisten the seed 
and to improve its con- 
dition for yielding oil dur- 
ing the pressing. 

3rd, Moulding. From 
the bottom of the kettle 
the hot seed is passed into 
a measuring box holding 
some 18 Ibs. of the crushed 
seed ; thence it passes to 
the table of a moulding 
machine, Fig. 58, where it 
is moulded into a cake of 
a certain size depending 

upon the size of the press plates, but usually having a thickness 
of H inch, the machine exerting a small amount of pressure to 
bring the mass of meal to this thickness. The surplus seed is 
returned to the kettle or the crushing rolls to be passed through 
with fresh seed. 




Fig. 58. Seed kettle and moulding 
machine. 



342 



PAINT VEHICLES. 



4th, Pressing. After leaving the moulding machines, the- 
cakes of meal are placed between two corrugated iron plates- 
hinged together like the backs of a book. In the Anglo- 
American process the use of hairs and cloths are dispensed with, 
thus effecting no little economy in the process of oil extraction, 

for the hairs and cloths 
were an expensive item in 
the older processes, as fre- 
quent renewals were re- 
quired. The press plates, 
besides being corrugated, 
are often marked with 
various letters and other 
marks, which, being im- 
pressed upon the cakes of 
meal, serve to show what 
firm pressed the oil cake. 
1 A number of the press 
plates and their contents 
are placed in the hydraulic 
press and subjected to a 
pressure of from | to'l ton 
on the square inch, for 
from 20 to 25 minutes, 
during which time most of 
the oil comes from the seed. 
It is a matter worth noting 
that the oil passes out from 
the seed cake along the 
edges and never from the 
sides of the cake. After 
being subjected to a pres- 
sure of about a ton for 25 
minutes, the pressure is in 
many works increased to 
about 2 tons, at which it 
remains for 5 to 7 minutes, 
then, when all the oil has 
flowed out of the seed, the- 
pressure is taken off, the press plates removed from the press and 
the oil-cakes taken out ; these are sold for feeding cattle, for 
which purpose they are of great value. The oil flows into tanks, 
from which it is pumped into large storage tanks, where it is- 
stored until required for further treatment. It is interesting 




Fig. 59. Hydraulic oil press. 



PROPERTIES AND COMPOSITION OF LINSEED OIL. 343 

to note that an intermittent pressure, such as is given by the 
old stamper press and in the modern forms of hydraulic oil 
presses, gives far better results than a steady uniform pressure. 

Fig. 59 represents the form of hydraulic press made by 
Messrs. Rose, Downs & Thompson, Hull, to whom we are 
indebted for the various illustrations of the crushing of linseed. 

5th, Refining. This is done in exactly the same way as in 
the English system, which see. 

Properties and Composition of Linseed Oil. Linseed oil 
is sold in two forms, known as " raw " and " boiled " linseed 
oil ; some text-books speak of a third form called " refined 
linseed oil," but the author considers this to be only a variety 
of the raw oil. Linseed oil as it comes from the press is rather 
turbid, of a dark colour, and contains some albuminous and 
mucilaginous matter. Before it is fit for use as a paint oil 
these must be removed, which is done by the methods detailed 
above ; then it forms the " raw linseed oil " of commerce. The 
" refined " linseed oil is mostly prepared for artists' use, and is 
obtained from the raw oil in various ways. 

In some cases the oil is allowed to stand for some months to 
settle, and then the clear top oil is exposed in closed glass vessels 
to sunlight to bleach it. Sometimes a small quantity of litharge 
or acetate of lead is mixed with the oil, and then, after standing 
some time, the oil, which is clear, is run off and bleached as 
before ; in other cases metallic lead is placed in the oil and left 
in contact with it for months ; the lead seems to exert a kind 
of bleaching action, and at the same time causes the albuminous 
matter in the oil to settle out. 

It may be laid down as a principle in treating oils that the 
simplest plan is always the best ; in any case the use of too much 
chemical action should be avoided. 

Raw linseed oil is a yellowish oil, having a brown hue, and 
possesses a characteristic odour and taste unlike those of any 
other oil. It is perfectly clear and limpid at all ordinary 
temperatures, but when subjected to moderate cold it thickens 
slightly, and solidifies at a temperature below 27 C. The vis- 
cosity of linseed oil measured in Hurst's standard viscosimeter at 
70 F. is for the three principal varieties : 

Baltic oil, .105 

Black Sea oil, 108 

East India oil, 112 

The specific gravity of linseed oil at 60 F. (15*5 C.) averages 
0-935, but ranges from 0-932 to 0-937 ; Baltic oil is usually the 
heaviest. A sample of Baltic oil examined by the author had a 



344 PAINT VEHICLES. 

specific gravity of 0-9378, a sample of Black Sea oil a specific 
gravity of 0-9326, while a sample of East Indian oil had a specific 
gravity of 0-9339. At 212 F. (100 C.) linseed oil usually has a 
specific gravity of 0-8801. 

Linseed oil is soluble in about 40 times its volume of alcohol 
at ordinary temperatures, and 5 times its volume at the boiling 
point. It readily dissolves in ether, petroleum spirit, shale 
naphtha, turpentine, chloroform, and similar solvents. 

Sulphuric acid has a strong action on linseed oil, causing it to 
become thick and of a dark colour ; large quantities of sulphur 
dioxide are evolved, while the temperature of the mixture is con- 
siderably increased, the amount varying somewhat in different 
kinds of linseed oil. Thus, with Baltic linseed oil the author 
obtained an increase of 120 C., with Black Sea oil an increase of 
114 C., and with East India oil an increase of 106 C. 

The action of nitric acid varies with the strength of the acid ; 
a moderately strong acid converts linseed oil into a viscid 
yellowish mass, which is insoluble, or nearly so, in petroleum 
spirit or benzol ; while strong, fuming nitric acid often causes 
linseed oil to take fire. Nitrous acid does not give a solid elaidin 
with linseed oil. 

In glacial acetic acid it is readily soluble on warming, while 
the turbidity temperature ranges from 36 C. to 47 0.* according 
to the quality of the oil and the strength of the acetic acid. 

Linseed oil combines very readily with bromine and iodine, 
absorbing a larger proportion of these bodies than any other oil ; 
there are slight differences between the various kinds of linseed 
oil in the quantities of iodine and bromine that they will com- 
bine with, but it may be laid down as a rule that the better the 
quality of the oil, the more iodine or bromine will it absorb. Of 
iodine the average amount taken up is 156 percent, of the weight 
of the oil, while of bromine the average is 98 per cent. That is 
100 parts of linseed oil will combine with 156 parts of iodine or 
with 98 parts of bromine. 

The property which gives linseed oil its special value as a 
paint oil is that when exposed to the air it gradually becomes 
hard, "dries up," in doing which it takes up from the atmosphere 
a large proportion of oxygen, forming a new compound of a resi- 
nous character, the properties of which have never been fully^ 
investigated. In this power of combining with oxygen, linseed 
is distinguished very markedly from other oils, which have little 
or no power of combining with oxygen. W. Foxf gives the 

* Hurst. On Valenta's test for oils. Journ. Soc. Chem. Ind., Jan., 1887. 
t Oil and Colourman's Journal, 1884, p 234. 



PROPERTIES AND COMPOSITION OF LINSEED OIL. 345 

following as the number of cubic centimeters of oxygen absorbed 
by 1 gramme of various oils : 

Baltic linseed oil, 191 

Black Sea linseed oil, 186 

East Indian Calcutta oil, 126 

East Indian Bombay oil, 130 

American linseed oil, 156 

Brown rape oil, 20 

Colza oil, . 17'6 

Cotton seed oil, 24 '6 

Olive oil, 8-2 

Evidently, the quality of linseed oil depends very much upon 
its oxygen-absorbing powers ; thus, Baltic oil, which dries better 
than any other variety of linseed oil, takes up more oxygen than 
Black Sea or East Indian, which latter takes up the least, and is 
the worst variety of linseed oil known. American linseed oil is 
not equal to Black Sea oil in its drying properties, but it is much 
better than East Indian, owing, as is clear, to its greater absorb- 
ing power for oxygen. 

Raw linseed oil when exposed in the form of a thin film to the 
air, as it would be in painting, takes about two days to become 
thoroughly dry and hard ; the coat left is firm and elastic, but 
has not much lustre or gloss. 

When linseed oil is heated with caustic soda or caustic potash, 
it undergoes saponification, the amount of alkali required being 
from 13-4 to 14 per cent, of caustic soda, and 18-7 to 19 '5 per 
cent, of caustic potash, while about 9*4 to 10 per cent, of glycer- 
ine is liberated. When the soaps so produced are treated with 
dilute sulphuric acid they are decomposed, and the fatty acids of 
the linseed oil are liberated. These acids have a specific gravity 
of from 0-924 to 0-927 at 15 0., and from 0-861 to 0-864 at 
100 0. They are solid acids, melting at 22 to 25 0., and solidi- 
fying at from 20 to 18 C., are insoluble in water, but readily 
soluble in alcohol, glacial acetic acid, ether and other similar 
solvents. Their combining equivalent is about 306, which points 
to the presence of acids of complex composition. 

Although the composition of linoleic acid (the characteristic 
acid of linseed oil) is usually given as C 16 H 28 O 2 , recent researches 
have thrown some doubt on the correctness of this formula. It 
may be pointed out that the combining equivalent of the fatty 
acids is 306, that is more than is indicated by an acid having 16 
atoms of carbon in its molecule. Allen has considered linseed 
oil to contain an acid which has 18 atoms of carbon ; he has 
named it homolinoleic acid, but gives no further details. If linoleic 



346 PAINT VEHICLES. 

acid contains 16 atoms of carbon, it is isologous with palinitia 
acid, and should on being hydrogenised, that is being acted upon 
in such a manner that it takes up hydrogen, yield palmitic acid ; 
while, as a matter of fact, it forms stearic acid, which is an acid 
containing 18 atoms of carbon. Then again linoleic acid when 
subjected to the action of alkaline permanganate of potash yields 
sativic acid, which is an hydroxy acid having the composition 
shown by the formula C 18 H 32 O 2 (H0) 4 . According to more 
recent researches linseed oil contains two acids. One is named 
linolic acid, and has the composition C 18 H 32 O 2 . It is a tetrolic 
acid, being capable of combining with 4 atoms of bromine. This 
acid yields sativic acid on oxidation with alkaline permanganate 
of potash. The other acid has received the name of linolenic 
acid, has the composition 18 H 30 O 2 , and belongs to a series of 
acids having the general formula C n H 2n 6 O 2 , a series not 
hitherto described. This acid has a high iodine value 245, as 
might be expected from its being capable of combining with 6 
atoms of iodine, or the same number of atoms of bromine. 

BOILED LINSEED OIL. The property which gives linseed 
oil its peculiar value to the painter is that it absorbs oxygen from 
the atmosphere when it is exposed in thin films or even in large 
masses, thereby forming the body known as oxylinoleic acid; this 
is capable of further oxidation to a body known as linoxyn, which 
has a composition indicated by the formula C 32 H 54 O n , and has 
some valuable properties, being quite neutral in its reactions, 
more or less transparent, and somewhat elastic. It is quite 
insoluble in water, alcohol, or ether, but is slightly soluble in 
chloroform. By long boiling with caustic potash it is saponified, 
forming a red soap. This body is the final oxidation product of 
linseed oil when exposed to the air. Oxidised oil can, by means 
of solvents, be separated into two products ; one^is insoluble, and 
the other is soluble. The insoluble body is, when freshly pre- 
pared, colourless, transparent, and gelatinous ; when dried it 
becomes a friable yet elastic solid. The soluble compound 
forms a coloured sticky mass resembling indiarubber, and for 
which it might act as a substitute. This property of absorbing 
oxygen is increased by heating the oil to a temperature of 400 
or 500 F. for a few hours ; it is still further increased by 
adding to the oil while it is being heated certain bodies which 
are known as "driers" (seep. 384). Oil'Vhich has thus been 
heated is known as boiled oil, because it is usually heated in 
large boilers. 

Boiling Linseed Oil. The boiling of linseed oil is done in 
several ways ; 1st, by fire heat ; 2nd, by steam. 



BOILING LINSEED OIL. 



347 




B. 



Fire boiling of Linseed Oil. The most usual or common method 
of boiling linseed oil is by means of fire in ordinary shaped boilers 
of capacities varying from 100 to GOO gallons. In shape these 
boilers are similar to those used in laundries, Fig. 60, A. One 
fault of these boilers is that they do not last very long, owing to 
the oil, or the acid in the oil, 
attacking and corroding the 
boiler. This action is most 
energetic at the bottom where 
the flame impinges, and is a 
very serious matter for those 
who use large and expensive 
boilers. An attempt was 
made to remedy this evil by 
making the bottom of the 
boiler thicker than the rest 
as in Fig. 60, B; but these 
did not last any longer, while 
a special evil was introduced, 
owing to the thick metal re- 
taining the heat so long that 
simple withdrawal of the fire 
did not prevent the oil boiling 
over, as it does in the case of 
the thin-bottomed boilers. In 
all large oil boiling works, the 
boilers are made of wrought- 
iron boiler-plate of the form 
shown in Fig 60, C; in this 
form the boiler is made of a 
uniform thickness of plate, 
and the bottom, which is 
made separately, is rivetted to 
the sides. As the corroding 
action is confined to the bottom 
and to the rivets, these can 
easily be replaced when re- 
quired \ so that these boilers 
will last longer than the other 

forms. The "boilers aVe set in the furnace in the usual way. 
It is best, however, in all cases, to set the furnaces against 
the wall of the boiling shed, and to place the fireplaces for 
feeding the furnaces outside the shed, so that, should the oil 
boil over during the process, there is less risk of a conflagration 




C. 



Fig. 60. Oil boilers. 



348 PAINT VEHICLES. 

from the oil finding its way into the fire. The fumes from the 
boiling oil should not be allowed to inconvenience the workmen, 
but should be conveyed to a hood placed over the boiler and 
connected with the chimney of the works. 

The boilers should be about one third larger than the quantity 
of oil which they are to boil, so as to allow room for expansion of 
the oil, and for any frothing or effervescence of the oil which may 
take place. 

The process of oil boiling is comparatively simple in principle, 
yet some difficulties crop up now and again in practice ; some of 
these are due to the quality of the oil, others are due to various 
obscure causes. 

The oil is placed in the boiler, which should never be more 
than two-thirds full, and the fire lighted. While the temperature 
of the oil is rising, the fluid should be closely watched as it is 
then that effervescence is likely to take place, and the oil to boil 
over with possibly disastrous results. Should there be any sign 
of the oil boiling over to too great an extent, then the fires should 
be withdrawn, and, by beating the oil, or ladling it out into an- 
other boiler, efforts should be made to keep down the effervescence. 
Much of this is due to the presence of small quantities of water 
and mucilaginous matter in the oil, and it more often occurs with 
fresh pressed oil than with oil which has been kept for some time 
after pressing, and from which, therefore, the water and mucilagi- 
nous matter has had a chance to settle out. After some time, 
dependent upon the quantity of oil being treated, and the size 
of the fire, the oil begins to enter into quiet ebullition ; this is 
said to be the boiling of the oil ; it occurs usually at a tempera- 
ture of about 500 F. The heating of the oil to this temperature 
should not be too rapid, so as to give the oil every chance of 
becoming oxidised, and it should not take less than two hours ; 
a longer time is preferable. When the oil has reached the boil, 
or better, after it has been boiling for about half an hour, a 
small quantity of driers is added ; other additions are made at 
short intervals during a period of three hours. The total amount 
of driers added varies a little in different works, but it averages 
about 5 Ibs. to 1 ton of oil. After all the driers have been 
added, the oil is boiled for one hour longer ; then the fire is 
drawn and the oil allowed to stand over night to cool and settle. 
The clear oil at the top is sent into the warehouse, and sold as 
"boiled oil," while the turbid oil at the bottom is known as 
boiled oil foots, and is used in making putty or putting into cheap 
ready mixed paints. It is not advisable to add the whole of the 
driers, small as it is in proportion to the oil, to the latter all at 



BOILING LINSEED OIL. 

once, as the action between the two might become too great, and 
the oil enter into rather violent ebullition, which could not be 
controlled readily ; by adding in small quantities at a time the 
action between the driers and the oil is less energetic and the 
boiling more under control; besides that, the combination between 
the driers and the oil is more complete. 

During the process of boiling oil, the latter undergoes some 
decomposition. Water is continually being given off, while large 
quantities of acrolein, C 3 H 4 O, a derivative of glycerine, which 
has a powerful action on the lachrymal glands of the eyes, acetic 
acid, formic acid, and other acids are also given off. As these 
products are somewhat obnoxious to the workmen, they should 
be conveyed by means of a collecting hood into the chimney of 
the works. The oil acquires a dark red colour, due to the 
presence of some of the products of the decomposition of the oil, 
although much depends upon the temperature at which the 
boiling is done. If this is kept below 400 F., a comparatively 
pale coloured oil can be produced; but if allowed to get over 
500 F., then a dark coloured oil is sure to result. The demand 
is now for a pale oil, so that in boiling it is desirable to keep 
the temperature down as low as possible. The quantity and 
nature of the driers used have some influence on the colour of 
the oil. Manganese produces a darker oil than any other drier ; 
next to this is red lead and litharge. The acetates of lead and 
manganese, and the oxalate of the latter metal, produce the 
palest oils. 

It will be found best to give an hour's extra boil at a low 
temperature, say from 400 to 450 F., rather than to heat the 
oil to 500 F. and over, when darkening is sure to occur. 

What the character of the action is which goes on during the 
process of boiling linseed oil is somewhat uncertain. That 
oxidation occurs is certain, but that is all that is definitely 
known; probably linoxyn, which may be regarded as the resin 
of linseed oil, is formed to some extent. Then, when driers are 
used, there is formed a combination of linoleic acid with the 
base of the driers, which, dissolving in the rest of the oil, forms 
a kind of varnish, to which action some of the gloss of boiled oil- 
is due. 

The function of driers in oil boiling is undoubtedly twofold. 
First, they act by increasing the oxidation of the oil, not by 
yielding up oxygen directly to the oil, because they are added 
in too small a quantity to have any appreciable action in this 
respect, but by acting in what is called a catalytic manner, causing 
by their presence a more ready and complete combination be- 



350 PAINT VEHICLES. 

tween the oil and the oxygen of the air. How they accomplish 
this is not known with certainty, and only the barest assumptions 
can be made on this point. It is noticeable that the best driers 
are compounds of bases which can form more than one oxide or 
more than one series of salts. Lead forms four oxides, man- 
ganese forms three well-defined oxides, iron three oxides, while 
each forms more than one salt with an acid. When such is the 
case, it often happens that the salts can easily be transformed 
from one kind to another. Thus, ferrous salts are readily 
changed into ferric salts and vice versd; manganous salts into 
manganic salts, and so on. When iron or manganese is used as a 
drier in oil boiling, it can be conceived that these changes are 
continually going on, the iron or manganese and lead, if the 
latter be used, passing from one stage to the other, carrying 
oxygen from the air to the oil; at the most, however, this is 
only a theory, and without many facts to support it. The 
author is not inclined to lay much stress on this theory of the 
catalytic action of driers. 

The decomposition of linseed oil is not complete, because some 
eight or nine per cent, of glycerine can be extracted from boiled 
oil. 

2nd. Steam Process of Oil Soiling. Besides the ordinary 
process by means of fire-heat, oil can be boiled by means of 
steam-heat. 

Vincent* describes the following arrangement for carrying 
out this process. A copper pan (Fig. 61) of sufiicient size is 
provided. This is fitted with a jacket, A, for steam to about 
two-thirds its depth. Above the pan, and forming a con- 
tinuation of it, is a dome, in which are three openings, one, B, 
in the centre for a vertical shaft, S, carrying agitators, C, for 
the purpose of keeping the oil in constant agitation during the 
operation. A large opening, D, in the front serves for the 
purpose of introducing the oil, and observing from time to time 
the progress of the operation, and for introducing, as required, 
the driers which are added to the oil. The third opening at the 
back is fitted with a flue, F, which passes into the chimney, 
and which serves to carry off any vapours which may be 
produced. Into the jacket a pipe, T, conveys steam at a pressure 
of 40 Ibs., while another pipe, L, connected with a pump passes 
into the oil chamber, and is for passing air into the oil. 

A very convenient size for such a boiler is one holding about 
2 tons of oil. 

* Journal Society of Arts, 1871. 



BOILING LINSEED OIL. 



351 



The plan of working is to first heat the oil in a tank with a 
closed steam coil for about two hours to a temperature of 
95 to 97 C. (203 to 206 F.). This preliminary heating prevents 
a great deal of frothing in the boiling pan, which is a matter of 
consequence, especially with Indian oils. 

From the tank the warm oil is run into the boiler and the 
steam turned on in the jacket ; after some time the oil begins to 
emit a peculiar, somewhat sickly odour ; then the air is blown 




Fig. 61. Steam oil boiler. 

through the oil, when the latter begins to froth a good deal and 
the odour increases and becomes more pungent. The agitators 
are kept at work during the whole of the time the oil is being 
treated. Driers are now added in small quantities at a time 
until ^ Ib. for every cwt. of oil in the pan has been added, this 
addition taking about two hours ; after which the oil is kept 



352 



PAINT VEHICLES. 



boiling for four hours longer. Then the steam and air is stopped 
and the oil run into the settling tanks, where it is allowed 
to remain for from three days to a week to settle. The 
clear oil is used for making paints, &c., and the foots for 
putty making. Another steam oil boiler is shown in 






















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Fig. 62. Oil boiler. 

Fig. 62, and is made by Messrs. Rose, Downs & Thompson. It 
consists of a jacketted still-pan set on brickwork ; in this an 
agitating gear is fixed, which may either be driven from a 
separate engine (as shown in the drawing) or from the general 
shafting of the works. The oil is placed in the pan, the steam 
turned on and the agitator set in motion ; when all effervescence 



ADULTERATION OF LINSEED OIL. 353 

of the oil has ceased driers are added in small quantities at a 
time, and the air is blown through by means of an air-pump. 

The advantages of using a steam process of oil boiling are that 
the risk of fire is greatly reduced, and the oil is paler in colour, 
which is a great consideration, although the character of the 
driers used will have some influence on this point, acetate of lead 
and other colourless driers giving the palest oil. For all purposes 
steam-boiled oil is as good as, if not better than, fire-boiled oil. 

Messrs. Hartley & Blenkinsop have patented the production of 
a heavy drying oil from linseed oil by the following process. A 
manganese linoleate is made by preparing a soap of linseed oil 
and adding this to a solution of manganese salt. The manganese 
linoleate is dissolved in twice its weight of turpentine, and from 
2 to 5 volumes of this solution are added to 100 volumes of 
linseed oil. Insoluble matters are separated, the mass raised to 
a temperature of 212 F. in a special apparatus, and a current of 
air or oxygen passed through the oil until it has attained the 
desired degree of thickness ; for example, a clear, transparent oil 
of a pale amber colour having a specific gravity of 0-997 can be 
obtained from a linseed oil of specific gravity 0*937. This thick- 
ened oil may be used in painting, and in the manufacture of 
floor-cloth, linoleum, and similar materials. 

Properties of Boiled Oil. Boiled oil is a slightly viscid 
oil of a reddish colour, varying a little in depth of colour 
according to the temperature, and the length of time it has been 
heated in the process of boiling. Its odour is peculiar, and its 
taste, which is rather acrid, somewhat characteristic. In specific 
gravity it varies a good deal, but the average is about 0-945 ; 
some samples will reach 0*950, while others may be as low as 
0-940. Boiled oil is soluble in turpentine, petroleum spirit, 
shale spirit, benzene, carbon bisulphide and other similar solvents. 
When boiled with caustic soda or caustic potash it is saponified 
almost completely ; there is usually a small trace of unsaponifiable 
hydrocarbon oil formed by the decomposition of the oil during 
the process of boiling. 

When exposed to the air in thin layers it dries much more 
rapidly than raw linseed oil, and leaves behind a hard, lustrous 
coat; it is this property which makes boiled oil of so much use 
to the painter ; yet it does not do to use boiled oil alone in the 
making of paints, because the coat which it leaves is too hard 
and rather liable to crack on exposure to the air; raw linseed oil 
is always added, as, by leaving a more elastic coat, it prevents 
this bad fault of boiled oil from showing itself. 

Adulteration of Linseed Oil. Both the raw and boiled 

23 



354 PAINT VEHICLES. 

linseed oils are frequently adulterated; (substitutes for boiled 
oil will be described more fully later on) ; the principal adul- 
terants used are mineral and rosin oils. Other fatty oils, 
such as cotton seed, niger seed, and whale oils, are sometimes 
used ; but, as linseed oil is cheap, the small gain arising from 
their use does not compensate for the probable loss of custom 
which must ensue if the adulteration be found out ; while the 
great difference in the cost of linseed and mineral oils is a strong 
inducement for adulterating with the latter. 

For the purpose of detecting adulteration the following tests 
may be applied : 

1. Specific Gravity. For raw linseed oil this should be about 
O f 932 ; if less than 0-930, adulteration with fish, seed, or mineral 
oils would be indicated ; while if the gravity exceeds 0-937, then 
admixture with rosin oil is very likely. The specific gravity of 
boiled oil averages about 0-945; if much heavier than this it is 
quite probable that rosin oil has been mixed with the oil ; while 
if below 0-940, then other fatty and mineral oils may be looked for. 

2. Flash Point. Linseed oil, whether raw or boiled, flashes 
at about 470 F. Other fatty oils flash at about the same tem- 
perature. Rosin oil flashes at from 300 to 330 F., and during 
the process of testing a strong odour of rosin would be given off'. 
Mineral oils, such as would be used to adulterate linseed oil, 
will flash at from 380 to 420 F., so that the flash point is one 
of the best tests for detecting the adulteration of linseed oil with 
mineral or rosin oils. 

3. Proportion of Mineral or Rosin Oils in Linseed Oil. To 
determine the proportion of mineral or rosin oils, in adulterated 
linseed oils, place 20 grammes in a beaker with a little water 
and alcohol ; then add some caustic soda and boil for some time, 
.stirring at intervals ; the linseed oil becomes saponified, while 
the adulterants are not acted on ; after about an hour's boil, the 
mass is allowed to cool a little ; then it is poured into a separating 
funnel and some petroleum ether is added, which will take up 
the mineral oil and form a layer on the top of the aqueous layer ; 
.after allowing the two layers to separate completely, the bottom 
layer is run off, and the top layer is washed quite free from all 
traces of the soap formed by the action of the alkali on the 
linseed oil, by several treatments with warm water. The ethereal 
layer is then run into a weighed glass, the ether evaporated off, 
.and the residue of mineral oil weighed. Whether the residue 
is mineral or rosin oil must be judged from the nature of the 
residual oil after evaporating off the ether ; if this is heavy and 
viscid, and smells of rosin when heated, then rosin oil is present ; 
if the residual oil is light, then mineral oil is present. 



POPPY OIL. 355 

4. Cotton and other Fat Oils in Linseed Oil The detection of 
cotton seed, niger seed, or other fat oils in linseed oil is much 
more difficult, but much valuable information on this point will 
be gained by noticing the behaviour of the oil with strong sul- 
phuric acid, the character of the mass formed, and the temperature 
which the mixture of acid and oil attains. The character of the 
soap formed on boiling the oil with caustic soda, the appearance, 
melting point, and combining equivalent of the fatty acids which 
may be obtained from the soap so formed are also valuable indi- 
cations of the character of the fatty oil adulterants. 

5. Driers in Boiled Oil. About 25 grammes are boiled with 
a little dilute hydrochloric acid, with constant stirring, for about 
half an hour ; the mass is allowed to stand to separate ; the 
bottom acid layer contains the driers added during the boiling 
of the oil, this is run off and tested in the usual way ; then the 
oil is boiled with caustic soda until it is saponified, then the 
mass is treated in the separating funnel, as described above, to 
separate the mineral or rosin oil used to adulterate the boiled 
oil. The aqueous layer which has been run off may be acidified, 
and the acids obtained tested for rosin by Gladding's test. 

BOILED OIL SUBSTITUTES. Many substitutes are 
offered for boiled oil, some of which have been patented. In 
composition they vary greatly, and it is not possible to do more 
than briefly indicate their general features. Some are mixtures 
of boiled oil, rosin, turpentine, rosin oil \ others more closely 
approach an oil varnish in composition, being made by melting 
rosin, then mixing it with hot oil and thinning down with rosin 
spirit. Some are made by preparing a compound of lime or 
alkali with rosin or other resinous products, and dissolving this 
in oil and rosin spirit or turpentine. 

The quality of these products varies very much. None of 
them are equal to good boiled oil, although one or two very 
nearly approach it; others are but inferior substitutes, and 
cannot be recommended even for inferior work. It is not 
possible to deal more particularly with these boiled oil substi- 
tutes in this book. 

POPPY OIL. This oil is obtained from the seeds of the poppy 
(Papaver somniferum) by pressure, or it may be extracted by 
means of solvents. This oil, although a very good drying oil, is 
not largely used, chiefly because its price does not allow it to 
compete with linseed oil ; artists make use of it on account of 
its paleness in colour not interfering so much with pale tints 
as linseed oil does, its price not being so much an object with 
them as it is with house painters. 



356 PAINT VEHICLES. 

Poppy oil is usually of a pale straw colour, very limpid, has 
little or no odour when fresh, and a pleasant taste ; the oil is 
free from the narcotic properties for which the plant itself is 
famous. In specific gravity it ranges from 0*924 to 0*927. It 
solidifies at - 18 0. It is soluble in about four times its volume 
of boiling alcohol and twenty-five times its volume of cold alcohol. 
Mixed with strong sulphuric acid (Maumene's test), the rise in 
temperature is about 88 to 90 0. It takes about 19 per cent. 
of caustic potash (K H) to saponify it, and it absorbs about 
134 to 137 per cent, of iodine. 

HEMPSEED OIL. The hemp plant (Cannabis sativa) yields 
a roundish greenish-grey seed, very familiar to lovers of canaries, 
from which, on expression, an oil is obtained that is used for 
painting. The yield of oil varies from 15 to 25 per cent. 

Hempseed oil when fresh has a greenish-yellow tint, but on 
keeping it slowly turns to a brownish-yellow ; its odour and 
taste are rather unpleasant. Its specific gravity ranges from 
0*925 to 0*931. It becomes turbid at a temperature of - 15 C., 
but does not set completely solid until a temperature of - 25 C. 
is attained. Strong sulphuric acid has a vigorous action on it, 
the increase in temperature being about 100 C. It absorbs from 
143 to 144 per cent, of its weight of iodine, which indicates that 
it contains a large proportion of linoleic acid, and shows that its 
drying properties must be good. 

In this country hempseed oil is rarely used as a paint oil, its 
price being against it ; still, it has been mixed with linseed oil, 
and it is difficult to obtain the latter free from it, owing to the 
Russian linseed growers mixing hempseed with the linseed. In 
Russia, and other places where hempseed is grown, the oil is 
used rather largely for painting. 

WALNUT OIL. The common walnut, the fruit of the 
walnut tree (Juglans regia) contains about 50 per cent, of its 
weight of an oil possessing drying properties. The process of 
extraction of this oil is as follows : 

The nuts are collected and placed in heaps for a period of about 
three months, when they begin to decompose ; they are then 
crushed and pressed ; this gives " virgin nut oil," often used as 
a food oil as well as a paint oil. The nuts still contain some 
oil, which is extracted by grinding the cake with hot water and 
again subjecting it to pressure; the oil so got is known as "fire- 
drawn nut oil." 

Walnut oil is usually of a pale yellowish-green tint, but it can 
be prepared from fresh kernels almost colourless. The specific 
gravity varies from 0*925 to 0*927 ; it begins to be turbid at a 



ROSIN OIL. 357 

temperature of - 15 C., but becomes solid only when at a temper- 
ature of 27'5 0. Strong sulphuric acid causes the evolution 
of some heat, the increase in temperature being 101 to 103 C. 
It will absorb about 144 per cent, of iodine, pointing to its con- 
taining linoleic acid in large proportion. 

It is a powerful drying oil ; some authorities say that it is 
superior to linseed oil in this respect ; at all events it is quite 
equal to it in drying power. It is chiefly used by artists, as it is 
pale in colour, and can, by bleaching, be obtained almost colour- 
less. Its greater cost prevents its coming into extensive use as 
a substitute for linseed oil in house painting. 

ROSIN" OIL. When rosin (the solid residue left by turpen- 
tine after all its volatile spirit has been distilled off by the aid of 
steam) (see p. 362) is distilled in large iron retorts it is decom- 
posed, and five principal products are obtained: 1st, gas; 2nd, 
pyrolic acid, a watery liquid containing 10 to 12 per cent, of 
acetic acid ; 3rd, rosin spirit ; 4th, rosin oil ; and 5th, a residue, 
rosin pitch, which is left behind in the still. The proportions of 
these bodies obtained depends partly on the nature of the rosin 
and partly on the method of distillation. 

There are two principal methods of distillation. In one, the 
most used, the rosin is placed in large vertical stills of cast iron 
capable of holding about 2,000 gallons, the usual charge being 
70 barrels of 25 gallons capacity. The still is connected with 
worm condensers and receiving tanks. When heated by fire the 
various crude products named above pass over. The proportion 
of products to the rosin employed varies from time to time, but 
the following are the average quantities : 

Gas, ....... 5*4 per cent. 

Acid water, 2 - 5 ,, 

Rosin spirit, 3'1 ,, 

Rosin oil, 85'1 ,, 

Pitch, 3-9 

Sometimes the distillation is carried to dryness, when coke is 
obtained instead of pitch as the final residue; but, pitch being 
the more valuable product of the two, this is seldom done. 
hrThe operation of distilling takes from 50 to 60 hours, the usual 
length of time being 56 hours. 

The second method of distilling is similar to the first so far as 
the plant is concerned, but the distillation is carried out with the 
aid of superheated steam in addition to fire heat. The main 
products are the same, but there is a larger proportion of spirit 
and a smaller proportion of rosin oil ; the spirit averages about 
15 per cent, and the oil about 62 to 64 per cent, of the weight of 



358 PAINT VEHICLES. 

the rosin employed, while the amount of acid water is necessarily 
largely increased by the condensation of the steam passed into 
the still. This process is not much used. 

The rosin spirit is described more fully on p. 372 et seq. 

The crude rosin oil which comes from the stills varies in appear- 
ance and quality at various periods of the process ; and also 
according to the manner in which the operation is conducted. 
Two varieties of crude rosin oil are recognised; " hard rosin oil," 
which is chiefly obtained when the distillation is conducted 
rapidly, and is the product which comes over during the first 
stages of the operation ; and " soft crude rosin oil," which is the 
product obtained when the process is conducted slowly, and 
during the middle period of the distillation ; sometimes a 
" medium crude rosin oil " is collected as the final part of the oil 
to come over. 

" Hard crude " is a thick turbid oil of a dark red colour, used 
for mixing with coal-tar and paraffin greases for making lubri- 
cating greases with. As it, when exposed to the air, absorbs 
oxygen and resinifies somewhat, it has been used as a paint oil, 
but its use, for reasons which will be pointed out presently, is 
unsatisfactory. 

" Soft crude" is rather thinner than the last; is, if anything, 
lighter in specific gravity, and is more acid in character. Its chief 
use is for mixing with lime in the preparation of wheel greases. 
It does not dry as well as " hard " rosin oil. 

The specific gravity of the crude rosin oil varies from 0*996 to 
1*030; it has acid properties, due to the presence of an acid or 
acids, the nature of which is unknown. It takes 0*3 parts of 
sodium hydroxide to neutralise the acid in 100 parts of the oil. 
There is much, about 60 per cent., unsaponifiable oil in crude 
rosin oil. 

Crude rosin oil is refined by alternate treatments with 
sulphuric acid, caustic soda, and redistillation; the oftener these 
operations are repeated the purer becomes the refined rosin oil 
obtained; the refining can be carried so far as to yield a product 
of a very pale colour. 

Refined rosin oil is a viscid, oily liquid, varying in colour from 
dark red to pale yellow in the best refined samples; its specific 
gravity ranges from 0*980 to 0*995; it has a peculiar odour. Its 
taste, especially if there is a strong after-taste, is peculiar and 
characteristic. The cruder varieties have a bluish or bluish- 
violet bloom or fluorescence, which is less marked in the more 
highly-refined samples. 

The cruder varieties of refined rosin oil contain small quantities. 



TABLE OP DRYING OILS. 



359 



. 



Ml 



11 











360 PAINT VEHICLES. 

of free acid and some products capable of combining with caustic 
soda; the more highly refined products are almost, if not entirely, 
pure hydrocarbons. The flash point of rosin oil is about 320 F., 
and the fire point about 390 F. The odour on heating is 
characteristic of rosin oil. 

The cruder varieties of rosin oil possess feeble drying powers, 
but the more refined oils have none at all. Rosin oil possesses 
one very objectionable feature in its drying properties ; it dries, 
but in the course of a few weeks the coat becomes soft and tacky 
or gives again, as the painters say ; even if rosin oil paint has 
been used for a bottom coat and a good linseed oil paint for a top 
coat this defect will make itself apparent. No means are known 
by which this defect of rosin oil can be remedied or of increasing 
the drying power in any degree. The use of rosin oil in paints 
should, therefore, be avoided. 

The addition of rosin oil to linseed oil or other paint oils can 
be readily detected by the increase in specific gravity, the low 
flash point, and the odour of rosin on heating ; while the amount 
may be approximately ascertained from the amount of unsaponi- 
fiable oil left after boiling with caustic soda. 

The other oils are of but slight importance, and the preceding 
table will give all necessary information about them. 

TURPENTINE. Turpentine is the term which was origi- 
nally given to some resinous exudations from various species of 
pine and other coniferous trees, but, of late years, this term has 
been used to distinguish a volatile liquid obtained from the crude 
turpentines by distillation; formerly this liquid was known as 
" oil " or "spirits of turpentine," and occasionally it is now so 
named; it is also known shortly as "turps." The crude turpen- 
tines are but of small value commercially, and some are only 
used in medicine. There are many varieties, such as Venice, 
Strasburg, Canadian, Chian, Aleppo, <fec. Each of these has a 
soft resinous character and an aromatic odour; when distilled 
with steam they are decomposed into a volatile spirit and a solid 
residue, rosin or colophony. As these bodies are not used in 
making varnishes no further mention will be made of them. 

Under the term turpentine will be described the liquid spirit 
used by painters and varnish makers. 

There are three varieties of turpentine met with in the English 
market viz., American, French, and Russian turpentines. All 
these are derived from various species of pine trees. 

American Turpentine is derived from two or three species of 
pine, chiefly from the swamp or Georgia pine (Pinus australis), 
which grows in extensive forests in North and South Carolina, 



FRENCH TURPENTINE. 361 

Georgia, and Alabama, the former State being the largest 
producer of turpentine. From the loblolly pine (Pinus taeda\ 
turpentine is also obtained. 

In winter, which extends from November to March, gangs of 
men proceed to the forests for the purpose of collecting the resin; 
for this purpose the trees are boxed, that is, a cavity is cut into 
the side of the tree, about 1 foot from the ground; the boxes 
have a capacity of about 2 or 3 pints. Sometimes 3 boxes will 
be made in a tree, but care is taken not to touch the heartwood, 
as such a proceeding would certainly kill the tree. The upper 
part of the box is always kept free from resin, and is frequently 
chipped so as to expose fresh surfaces of wood, which causes the 
resin to flow more freely. About March, the sap begins to flow 
and to collect in the box and on the sides of the cut surface, that 
which collects in the box is called " dip," and that which collects 
on the sides is known as "scrape." That which collects the first 
year in a box is known as " virgin dip," and is always collected 
separately. The crude resin is known commercially as "gum 
thus," and is exported for use in making varnishes. Most of the 
resin is, however, treated locally for turpentine and rosin. 

Turpentine is obtained from the crude resin by placing it in a 
still; into this still passes a steam pipe from a steam boiler, 
while out of it passes a pipe in connection with a worm condenser; 
a manhole on the top serves for the purpose of filling the still, 
while a large pipe at the bottom serves to run off the residual 
rosin. The still is built into a suitable furnace, so that it can be 
heated by fire. When sufficient resin has been placed in the 
still the fire is lighted, and when the temperature has attained a 
little above the boiling point of water, the current of steam from 
the boiler is sent in; the turpentine passes over into the worm 
condenser and condenses along with water from the steam ; when 
no more turpentine comes over, the rosin left in the still is run 
off into barrels, when the still is ready for another charge. The 
turpentine is often purified by a second distillation. The appli- 
ances in use are generally of a crude description. 

The properties of American turpentine will be dealt with 
shortly. 

French Turpentine. This variety is obtained from the 
maritime pine (Pinus maritima), which grows very extensively 
in the South-west of France, especially in the Departments of 
Landes and Gironde. The industry in these districts is con- 
ducted on a rather more scientific principle than in America. 
The trees are cut in February or March, and the sap is caused 
to flow into an earthenware vessel placed at the foot of the tree. 



362 PAINT VEHICLES. 

The trees are tapped for five years in succession, when they are 
not touched for a few years, and then tapping commences again; 
when the tree has got somewhat exhausted, the final tapping 
takes place, and a large yield of resin obtained, but the tree is 
killed. It is felled, and another planted in its place. 

The distillation of the crude resin is carried on in the plant 
shown in Fig. 63, which is a front view or section of, and 
Fig. 64, which is a side view or section of the plant. A is a 
boiler heated by means of a steam coil, or (as shown in the 
drawing) by means of a fire, the former method being preferable ; 
in this, the crude resin is heated to a temperature of 96 0. 
(194 F.), when it becomes liquid. The boiler is fitted with a 
movable cover to prevent the easy escape of turpentine and the 
entrance of dirt. When the resin is melted, it is run into the 
tank, B, through the pipe, a; the particles of woody tissue, dirt, 
&c., are deposited in this tank, as also in the boiler, A. From 
the tank, B, the melted resin is run into a tank, C, which holds 
the quantity usually treated at one time (about 66 gallons); this 
tank is, therefore, a measuring tank, from this it runs through 
the pipe, b, into the still, D, which has the form shown; into this 
passes a steam coil, by which steam from an ordinary steam 
boiler can be sent into the still. An opening near the bottom 
of the still (which is kept plugged during the time the turpen- 
tine is being distilled), permits of the rosin being run off. E is 
an ordinary worm condenser fitted into a tub through which 
cold water is continually passing ; with this worm condenser the 
still is connected by means of a goose neck (shown in the 
drawing). 

The crude resin is placed in the still, 66 gallons being the 
usual charge; it is then heated by fire until a temperature of 
135 C. (275 F.), is attained; when a current of steam is passed 
into the still, turpentine begins to come over and to condense 
along with the water from the steam in the worm condenser, the 
condensed products passing into a suitable receptacle, in which 
the water gradually settles to the bottom, while the turpentine 
rises to the surface; the latter is skimmed off and run into 
other narrow-mouthed vessels, in which it is allowed to stand 
for several days, during which the remaining water and other 
impurities settle out. The yield of turpentine is rather more 
than one-fifth that of the crude resin employed. 

When all the turpentine has been distilled over, the residue 
in the still is run first into a tank, F, and from thence into a 
revolving screen, G, through which it flows in a fairly clear 
condition free from dirt and grit of any kind. By this means, 



TURPENTINE DISTILLING PLANT. 



365 




364 



PAINT VEHICLES. 




TURPENTINE. 36-5 

too, the rosin is freed from any water it may contain. The 
quantity of rosin obtained is rather less than four-fifths of the 
resin used. 

French turpentine is almost entirely consumed in France; 
very little is now exported into England. Its properties will be 
discussed later on. 

Russian turpentine is obtained chiefly from the Scotch pine, 
Pinus sylvestris. The method of obtaining it does not differ 
essentially from that adopted in extracting American or French 
turpentine, although there are some minor differences in the 
method of tapping the trees and collecting the crude resin, and 
in the manner of distilling the turpentine, which is usually done 
in a rather crude manner. 

Russian turpentine differs slightly in properties from American 
and French turpentines. 

Turpentine is a hydrocarbon having the formula C 10 H 16 ; 
there are, however, a number of isomeric compounds known 
which have the composition represented by the above formula. 
These bodies have been named the terpenes ; they are derived, 
as well as the three varieties of turpentine already described, 
from natural resins or from various natural oils. 

They closely resemble one another in their chemical as 
well as in many of their physical properties. The terpenes 
have been investigated by Berthelot, Tilden, Wallach and 
other chemists, and a number of them are known. Berthelot 
pointed out that French turpentine had some different pro- 
perties from American turpentine, although their chemical 
composition was the same. He named the terpene of American 
turpentine, australene, and that from French turpentine, 
terebenthene ; while he gave to the characteristic hydrocarbon 
of Russian turpentine the name of sylvestrene. Armstrong 
considers that American turpentine is a compound of two terpenes r 
one of which is the same as found in the French turpentine and 
which rotates a ray of polarised light to the left ; this he names 
Isevoterebenthene. The other terpene has similar properties, 
only it rotates the ray of polarised light to the right ; this he 
names dextroterebenthene ; it is found in a very pure condition 
in the turpentine from Pinus Khasyana, a Burmese tree. Wallach 
describes nine terpenes which he names 1. Pinene, the main 
constituent of French and American turpentine. 2. Camphene, 
which differs from all other terpenes in being solid ; it is not 
found naturally, but can be prepared by artificial means from 
pinene. 3. Fenchene, which is also obtained artificially. 4. 
Cimonene, found in the essential oils of various species of 



366 PAINT VEHICLES. 

aurantiacece, oils of lemon, orange, bergamot, &c. 5. Dipentene, 
found in oil of camphor, Russian and Swedish turpentine, <fec. 
6. Sylvestrene, the characteristic terpene of Russian and Swedish 
turpentine. 7. Phellandrene, found in various essential oils. 8. 
Terpinene, found in several oils. 9. Terpinonlene, a rare terpene. 
The two most important of these are Pinene and Sylvestrene, 
which are found in the chief commercial turpentines. 

Pinene is a colourless or water-white mobile liquid of a 
peculiar and characteristic odour, having a specific gravity of 
O-8749 according to Tilden; Wallach gives it as 0-860. It boils 
at from 155 to 156 C. When dry hydrochloric acid gas is 
passed into it, combination ensues, and a crystalline body having 
the formula 10 H 16 H01 is formed; this closely resembles camphor 
in appearance and is known as artificial camphor; by heating, 
under pressure, with caustic potash this body is decomposed and 
the solid terpene, camphene, is formed. When pinene is exposed 
to sunlight in the presence of water a crystalline compound is 
formed which has the composition 10 H 18 O 2 , and is named by 
Armstrong sobrerol. Pinene in contact with water gradually 
combines with it, forming a crystalline hydrate, terpene hydrate, 
C 10 H 16 3 H 2 O, which is soluble in alcohol, insoluble in turpen- 
tine, slightly soluble in cold water, a little more freely in hot 
water, and sparingly soluble in ether, chloroform and carbon 
bisulphide. 

There are two varieties of pinene, which differ from one another 
simply in their action on a ray of polarised light. One variety, 
that in French turpentine, turns the ray to the left, and is 
distinguished as laevo-pinene, the other is found in American 
turpentine, and turns the ray to the right, and is named dextro- 
pinene. The air-oxidation and other products from the two 
terpenes differ from one another in the same manner. A mixture 
of the two pinenes, in equal proportions, would have no action 
on polarised light, and gives rise to inactive oxidation products. 
American turpentine contains both pinenes, the dextro variety 
predominating. 

Sylvestrene is the characteristic terpene of Russian and 
Swedish turpentine, derived from the Scotch pine, Pinus sylvestris. 
It is a colourless, or water-white limpid liquid, having a specific 
gravity of 0*846 at 20 C.; it boils at 175 C. It has a dextro- 
rotatory action on polarised light; the Isevo-rotatory and inactive 
varieties are not known. Dry hydrochloric acid gas, when 
passed through Sylvestrene, forms an hydrochloride, C 10 H 16 HOI, 
which is liquid. In this respect this terpene differs from pinene; 
it is also more easily oxidised when exposed to air and light. 



TURPENTINE. 367 

The other terpenes are of no practical importance to the painter. 
Commercial French and American turpentine is a water- white, 
limpid liquid, with a peculiar and characteristic odour that dis- 
tinguishes it from all other bodies. The specific gravity ranges 
from 0-864 to 0-870, but usually is about 0-867. French turpentine 
is a little more uniform than American turpentine in this re- 
spect. It begins to boil at from 156 to 160 C., and is completely 
distilled at 170 C. If the sample be fresh, there is little or no 
residue left behind, but old samples generally leave a slight 
residue of resinous matter, which in any case does not amount 
to more than 1 per cent, of the turpentine. 

Turpentine is readily combustible, burning with a smoky 
flame, a peculiar and characteristic odour being evolved. The 
flashing point of ordinary turpentine is 36 to 38 C. (97 to 
100 F.). 

Turpentine is readily miscible with ether, carbon bisulphide, 
alcohol, benzene, petroleum spirit, but it is insoluble in water. 
It is a good solvent for oils, fats, resins, &c. 

On exposure to the air in bulk, turpentine absorbs oxygen 
slowly from the atmosphere, becoming thick and viscid or fatty 
in appearance. A prolonged exposure causes the turpentine to 
become resinous, part of the turpentine volatilising during the 
exposure. In thin layers, such as would be formed when turpen- 
tine is spread over a surface with a brush, a condition of affairs 
which occurs in painting, there is less oxidation, as a larger 
proportion of the turpentine volatilises away, and the oxidation 
of the residue is more complete, so that a hard resinous product 
is the result. This property distinguishes turpentine from all 
the other spirituous liquids used by the painter and varnish 
maker. These evaporate completely away, and consequently 
leave no residue behind which can act as a binding agent for 
the pigment or colouring matter of the paint; whereas, the resin 
left when turpentine is used, acts as a binding agent, and fixes 
or fastens the pigment of the paint on the surface over which it 
is spread. American has greater absorbing powers for oxygen 
than French turpentine. 

Exposed to the air in contact with water, turpentine forms a 
solid crystalline product, having the composition C 10 H 18 O a . 
This has been named sobrerol; the melting point is 150 C. for 
the active variety, and 130 -5 to 131 C. for the inactive variety. 
The crystals belong to the rhombic system, the inactive variety 
being of a different form to the active varieties. They are some- 
what soft and flexible, and are soluble in alcohol. 

When repeatedly distilled with strong sulphuric acid, turpen- 



368 PAINT VEHICLES. 

tine becomes polymerised. Generally two bodies are formed- 
One of these has been named terebene, which has the same 
formula (C 10 H 16 ) as turpentine, and, when pure, boils at 160 CX 
The other body has been named colophene, has the formula 20 H 32 , 
and boils at 300 C. It constitutes the main product of the reaction. 
This property of polymerisation, which is essentially a conversion 
from a spirit boiling at a low temperature into a spirit boiling at 
a high temperature, distinguishes turpentine from any of its 
substitutes. 

Nitric acid acts very energetically on turpentine, the result 
varying with the strength of the acid used. If strong enough, 
the turpentine may take fire; in any case, various oxidation 
products are obtained. 

Chlorine, bromine, and iodine act with great energy on turpen- 
tine ; great care must be taken in bringing these bodies into 
contact with one another or explosions may occur. 

Turpentine has a strong action on polarised light, a property 
which distinguishes it from benzene, petroleum spirit, and rosin 
spirit. French turpentine rotates the ray to the left, its specific 
value being - 30, and is fairly constant, showing that French 
turpentine has a very uniform composition; this fact is also 
shown by its regular specific gravity and by its steady distillation 
temperature. The specific rotation of the pure terpene, tereben- 
thene, is - 40. American turpentine rotates the ray to the right, 
but the variation of the value in different samples is very great ; 
ordinary commercial samples give specific values ranging from 
+ 8 to + 16 ; the pure terpene has a specific rotation of + 21 -5. 
It is quite possible that American turpentine contains a small 
quantity of a Isevo-terpene, the quantity of which varies in 
amount and, consequently, the specific rotation must vary also. 
The air oxidation products (sobrerol) in their action on polarised 
light vary with the turpentine from which they are obtained ; 
that from French turpentine rotates the ray to the left with a 
fairly uniform specific value, while that from American tur- 
pentine is rather variable and can be separated into two 
varieties, one with a + and the other with a - specific rotation; 
while, by mixing the two in equal proportions, an inactive 
variety can be obtained. Burmese turpentine from Pinus 
Khasyana, which resembles French and American turpentine in 
its general properties, differs by having a strong and uniform 
+ rotation. 

Russian turpentine resembles American turpentine in many 
of its properties, such as solvent and soluble features, action of 
nitric acid, sulphuric acid, chlorine, &c. It is rather more- 



TURPENTINE. 369 

variable in composition and specific gravity, which latter varies 
from 0-862 to as high as 0-873. It begins to boil at about 
156 C., but is not completely distilled below 180 C., the great 
bulk passing over between 172 and 174 C.; this greater range 
of distilling temperature points to a more complex composition 
than that of other turpentines. It has an odour resembling that 
of American turpentine, but differing slightly therefrom. It is 
rather more volatile. It rapidly absorbs oxygen from the at- 
mosphere, becoming very viscid ; partly on this account and 
partly on account of its stronger odour, Russian turpentine has 
not come so much into use in making paints. It is said to 
induce headache when being used ; this phenomenon will depend 
a great deal on the physiological idiosyncrasies of particular 
individuals. The oxidation-product which is formed has, accor- 
ding to Kingzett, the composition C 10 H 14 O 4 , and he has named 
it camphoric peroxide ; on heating with water, this gives rise to 
the formation of camphoric acid, C 10 H 16 O 4 , and hydrogen 
peroxide, H 2 O 2 . On this property of Russian turpentine is 
based its use in the preparation of the disinfectant, " Sanitas." 

Russian turpentine exerts a strong rotary action on polarised 
light, the specific value varying as much as from + 15 to + 23, 
while the pure terpene, sylvestrene, has a specific rotation of 
+ 19, which shows that the commercial turpentine must contain 
terpenes of higher rotary power, the amount of which varies in 
different samples. In all other properties Russian turpentine 
resembles American turpentine. 

Of late years a great many substitutes for turpentine have 
been placed on the market under a variety of fancy names, 
"patent turpentine," " turpentyne," " turpenteen," <fec. These 
will be discussed later on (see p. 371). 

Turpentine is frequently adulterated, the adulterants usually 
added being petroleum spirit, shale naphtha, rosin spirit, and 
coal-tar naphtha. The fact of the adulteration and the nature of 
the adulterant added is easily ascertainable, but the question of 
proportion of adulteration is a more difficult matter to ascertain, 
and demands the exercise of some considerable care and skill on 
the part of the analyst. 

The property of acting on polarised light distinguishes turpen- 
tine from all bodies used to adulterate it ; the specific value for 
each variety of turpentine has already been given ; the presence 
of other bodies tends to reduce these values in proportion to the 
extent of the adulteration. 

The specific gravity is a good indicator of adulteration, as, with 
the exception of coal-tar naphtha, the addition of other spirits tends 
to cause it to vary from the normal average of 0-867. 

24 



370 PAINT VEHICLES. 

The addition of any of the adulterants named has a material 
influence on the temperature at which the turpentine begins to 
boil and those at which it distils. Genuine turpentine does not 
begin to distil below 150 to 156 0., the thermometer rising slowly 
from this point. In the case of French or American turpentines 
all is distilled over before the temperature attains 170 C., while 
with Russian turpentine the temperature may reach 180 C. 
before all is distilled. The greater portion of the turpentine 
passes over between 158 and 161 C. with American or French 
turpentines, while at least 93 to 95 per cent, passes over below 
165 C. The great bulk of Russian turpentine passes over be- 
tween 170 and 175 C. Adulterated turpentine begins to distil 
much below 150 C., in some cases below 80 C., according to the 
nature of the adulterant added ; from the point at which the 
sample begins to distil there is a gradual increase of temperature; 
the thermometer rises slowly, and in some cases the distillation 
is not complete at 200 C. By comparing the temperature at 
which the sample distils with the distillation temperatures of the 
possible adulterants as given in the descriptions of these bodies 
some idea of the nature and the approximate amount added of 
the adulterant may be obtained. 

The simplest method of ascertaining the distillation temper- 
atures of turpentine is by means of a retort; this should have 
the capacity of 150 c.c. and be provided with a stopper. Through 
the tubulure is passed a thermometer; 100 c.c. of the turpentine 
is measured into the retort ; the bulb of the thermometer is com- 
pletely immersed in the liquid; while the beak of the retort is 
connected with a Liebig's condenser. A cylindrical measuring 
glass of 100 c.c. capacity serves as a receiver. Heat is applied to 
the retort, and the temperature at which the liquid enters into 
ebullition, and gives off vapour which condenses in the beak of 
the retort is noted; the rise of thermometer and the quantity of 
distillate collected in the receiver is also noted from time to time. 
The distillation need not be carried to complete dryness, but may 
be stopped when about 5 per cent, is left in the retort. 

The distillation may be carried out in a fractionating flask, 
which is a flask fitted with a tube in the side of the neck, which 
tube is connected with a condenser. Into the mouth of the flask 
is fitted a thermometer, the bulb of which is arranged to be 
opposite the side tube. 100 c.c. of the sample is measured into 
the flask and the sample distilled as before. More uniform 
results can be obtained by using a fractionating flask than with 
a retort, the thermometer registering more accurately the temper- 
ature of the vapour as it passes to the condenser. 

Petroleum or shale naphtha in turpentine may be detected by 



TURPENTINE SUBSTITUTES. 371 

the flash point, which in genuine turpentine is about 90 to 100 
R, while the naphthas flash at 60 to 70 F.; the addition of 
10 per cent, of these is suflicient to affect the flash point. A 
method of ascertaining the addition and the amount of these 
bodies can be based on the fact that while turpentine can be dis- 
tilled in a current qf steam, the shale and petroleum naphthas 
cannot. Into a retort is placed 100 c.c. of turpentine, which is 
gently heated by means of a Bunsen burner ; through the retort 
is passed a current of steam generated by boiling water in a flask, 
the steam from which is passed through the turpentine in the 
retort. The distillation is carried on until no more spirit passes 
over, a Liebig's condenser being used to condense the vapours 
which pass over. Under these conditions genuine turpentine 
does not leave more than 0'2 per cent, of unvolatile residue if 
fresh; old samples may leave as much as 0*5 per cent. Any 
residue above 0'5 per cent, which may be left in the retort may 
be considered to be evidence of adulteration, and its nature can 
be ascertained by a few experiments. Those portions of the 
naphthas from either shale or petroleum which are volatile below 
100 C. will pass over with the turpentine ; in this event the 
distillate will have a specific gravity below 0*800, as a rule. 

Rosin spirit is rather more difficult to detect when it is used 
for adulterating turpentine. It increases the specific gravity. Its 
wide range of distilling temperatures and its odour are sufficient 
to detect it, but there is no satisfactory method of determining 
the proportion which may have been added ; the steam distillation 
process gives the best results. 

Coal-tar naphtha is not much used for adulterating turpentine; 
it has much about the same specific gravity, flashes at about the 
same temperature, has a wider distilling range of temperature, is 
more volatile, and has a peculiar odour. 

A table will be found on p. 379 giving, in a comparative form, a 
synopsis of the properties of turpentine and its adulterants, which 
will be found useful in making the analysis of an adulterated 
sample. 

TURPENTINE SUBSTITUTES. A number of these 
bodies are now offered to painters under various fancy names, 
(see p. 369). The composition of these bodies necessarily varies ; 
some are simply heavy petroleum hydrocarbons of about 0*790 
specific gravity and flashing at about 120 F. An example of 
such will be found on p. 376. Others are mixtures of turpentine, 
rosin spirit, and benzoline, in various proportions, while others 
contain only rosin spirit and petroleum light oils. 

The properties of these bodies will be those of their various 



372 



PAINT VEHICLES. 



constituents, and these will be found described in their proper 
places. They are more or less efficient substitutes, but whether 
they are good or bad depends entirely on their composition ; none 
are equal to turpentine for paint-making. 

ROSIN SPIRIT. When rosin (see p. 362) is subjected to 
distillation (p. 357), either by fire heat alone or with the aid of 
superheated steam, there is obtained, as one of the products, a 
light volatile spirit, which, when crude, has a dark brown colour. 
As a rule, the proportion of rosin spirit obtained is small ; when 
fire heat alone is used only about 3 per cent, is obtained; if super- 
heated steam is employed from 10 to 15 per cent, is obtained. 
This rosin spirit is refined by treating, first with sulphuric acid, 
next with caustic soda, and then redistilling the washed spirit. 

Rosin spirit is a limpid, water-white liquid ; the colour varies 
with the degree with which the refining has been done. It has 
a peculiar and characteristic terpene odour. Its specific gravity 
varies considerably, from 0-876 to 0'8S3, but it is invariably 
heavier than turpentine. Exposed to the air, it volatilises in 
part and partly oxidises, the result of the oxidation being the 
formation of a resinous residue, as is the case with turpentine, 
but not to the same extent. It flashes at from 36 to 38 C. 
(97 to 102 F.). 

On heating in a retort or flask, rosin spirit enters into ebulli- 
tion and distils over ; the temperature rises during the whole of 
the time of boiling. The temperature when boiling begins, the 
rapidity of distillation, and the temperature below which it is all 
distilled over varies much. A sample examined by the author 
gave the following figures : 





5 per cent, came over below . 








127 C. 


8 per cent, more came over below 








137 C. 


11 














149 C. 


34 














159 C. 


184 














169 C. 


2 














179 C. 


13 














190 C. 


6 














200 C. 


8 














220 C. 


6 














237 C. 



When the operation was stopped, 18-25 per cent, remained in 
the flask ; this is rather a heavy sample ; some samples will distil 
more completely. It may be laid down as a rule that a spirit 
completely distilling below 240 C. is better for use in paint- 
making than one distilling above that temperature. The con- 
tinual increase of temperature during the process of distillation 



EOSIN SPIRIT. 373 

is a point of distinction from turpentine, and points to complexity 
in the composition of rosin spirit. 

Rosin spirit is insoluble in water and alcohol, but is soluble in 
ether, or a mixture of alcohol and ether, as also in turpentine, 
chloroform, and petroleum spirit. Nitric acid acts on it rather 
less energetically than on turpentine, forming with it a dark 
scarlet mixture ; hydrochloric acid has but little action. Sul- 
phuric acid forms a dark red mixture ; probably there is some 
polymerisation. 

In composition rosin spirit is a mixture of several hydro- 
carbons the exact nature of which has not yet been thoroughly 
worked out. From rosin spirit has been isolated heptine, 7 H 12 , 
a colourless, limpid liquid, having a specific gravity of 0*8031, 
and boiling at 103 to 104 0.; on exposure to the air it absorbs 
oxygen. Sulphuric acid polymerises it with the formation of 
diheptine, C 14 H 24 , which boils at from 235 C. to 250 C., and 
which, on exposure to the air, rapidly absorbs oxygen and resini- 
fies. Heptine exposed to air in the presence of water forms 
crystals having the formula O r H 12 (O H) 2 H 2 O. It combines 
with bromine to form the compound 7 H 12 Br 2 , which is a heavy 
yellow oil. In light rosin spirit, containing constituents boiling 
below 100 0., there have been found hexylene, C 6 H 12 , and amy- 



Rosin spirit is the best substitute for turpentine known, and 
is capable, when of good quality, of replacing it for all purposes. 
The chief objection to it lies almost entirely in its odour, which 
is not so pleasant as that of turpentine. Then, again, if sufficient 
care has not been taken in refining it, it is apt to contain traces 
of rosin oil, which would prevent it from drying properly. The 
specific gravity of a good sample of rosin spirit should not exceed 
0-880, and all should distil below 250 C. ; it may be taken for 
granted that any residue left at temperatures above 250 0. 
will consist chiefly of rosin oil, which reduces the value of the 
rosin spirit, owing to its want of drying properties. Rosin 
spirit is used in making some of the turpentine substitutes 
which are now so common. 

The only possible adulterants for rosin spirit are shale 
naphtha and petroleum benzoline the presence of which can 
be ascertained by the application of the tests for specific 
gravity, flash point, and distillation temperatures, as in the 
case of turpentine. 

Rosin spirit is largely used in making cheap varnishes, using 
rosin (as the body), gum, and, occasionally, colouring the varnish 
with pigments. 



374 



PAINT VEHICLES. 



SHALE SPIRIT or SHALE NAPHTHA. When the 

shale found in the South of Scotland, in the district lying 
between Glasgow and Edinburgh, is distilled at a comparatively 
low temperature, there is given off a quantity of tarry matter 
known as "crude oil;" this is a product of very complex 
composition, which, on redistillation, gives a purer product 
known as "once-run oil." This is further refined by treatment, 
first, with sulphuric acid (to remove basic impurities), and 
then with caustic soda (to remove acid impurities) ; finally, it is 
distilled, when it yields three chief products "green naphtha," 
" twice-run light oil," and " green oil." The two latter do not 
interest painters. 

The "green naphtha" is refined by treatment with sulphuric 
acid, caustic soda, and redistilling, when there is obtained a 
very limpid, water-white liquid, known commercially as " naph- 
tha" or "shale spirit"; the yield is about 5 per cent, of the 
crude oil. 

Shale naphtha is a water-white, very limpid liquid, having a 
specific gravity of O730 to 0-760, and a slight odour. It is 
insoluble in water and alcohol, but mixes freely with ether, 
turpentine, benzol, &c. ; while it readily dissolves all oils- 
(except castor oil), some of the resins (such as gum dammar) in 
the natural state, and nearly all when they have been partially 
decomposed by fusion. It is very volatile, and, unlike either 
turpentine or rosin spirit, evaporates without leaving any 
residue behind. 

It is inflammable, readily taking fire at ordinary tempera- 
tures, which is one objection to its use. It begins to boil below 
100 C., and is generally completely distilled over below 190 C. ; 
the range of temperature and the proportion which distils over 
varies with different makes. A sample (sp. gr., 0*760) tested 
by the author distilled as follows : 

First drop at . . . 71 C. 

3 per cent, came over below 90 C. 

10 120 C. 
23 



43 
60 
70 
81 
90 



143 C. 
150 C. 
160 C. 
170 C. 
180 C. 
190 C. 



A naphtha of 0-730 specific gravity will boil at lower tempera- 
tures than these. 

Acids and caustic alkalies have no action on shale spirit. It 
absorbs a small quantity of bromine or iodine. 



BENZOLINE, BENZINE, OR PETROLEUM SPIRIT. 



375 



Shale naphtha is a complex mixture of hydrocarbons belong- 
ing to the two series, paraffins and olefines, the latter forming 
about 60 per cent, of the naphtha. There have been found 
hexene, C 6 H 12 ; heptene, C 7 H 14 ; octene, C 8 H 16 ; and nonene, 
C 9 H 18 among the defines : hexane, C 6 H 14 ; heptane, C 7 H 16 ; 
octane, 8 H 18 ; and nonane, C 9 H 20 among the paraffins ; but 
other members of these two series are present. 

Shale naphtha is largely used as a substitute for turpentine, 
and on the whole is a good material for the purpose. It is 
distinguished from both turpentine and rosin spirit by its much 
lower gravity, its flash point, its indefinite boiling point, the 
low temperatures at which it distils, by not being affected by 
sulphuric acid, and by not being completely distilled in a 
current of steam as is turpentine. 

BENZOLINE, Benzine, or Petroleum Spirit. When the 
petroleum obtained from the oil wells of North America is 
subjected to distillation three products are obtained viz., 
"naphtha," "kerosine," and "residuum." 

The " naphtha " is refined by treating with sulphuric acid, 
then with caustic soda, and distilling, when three products are 
obtained viz., "gasoline," "benzoline," and "naphtha." The 
first gasoline is a very light product, having a specific 
gravity varying from 0-680 to 0*700, and is used only for 
special purposes. The other two products are sold indiscrimi- 
nately under the names of "benzoline," " benzine," and "petro- 
leum spirit." These products vary much in quality. They 
are water-white, very limpid liquids having a specific gravity 
ranging from 0*730 to 0-760, but heavier samples are met with. 

In its general properties benzoline resembles the shale 
naphthas ; what is generally sold under that name has a specific 
gravity of about 0-730 ; it flashes and takes fire at the ordinary 
temperature ; it begins to distil at about 65 C., and is usually 
completely volatilised at 150 0. 

A sample tested by the author gave the following results : 



First drop came 
5 per cent. 
14 


over at 
bel 




65 C. 
70 C. 
75 C. 


ow 








22 














80 C. 


31 














85 C. 


46 














90 C. 


58 














95 C. 


64 














100 C. 


70 














105 C. 


80 














110 C. 


90 














120 C. 



376 PAINT VEHICLES. 

This is a light sample ; usually only about 50 per cent, distils 
over below 100 C.; and some samples do not contain any portion 
distilling below 100 C. 

In its general features, chemical composition, and uses, 
petroleum spirit resembles shale spirit ; it contains, however, a 
larger proportion of the paraffin hydrocarbons. 

Certain substances are met with in the market, and sold as 
"turpentine substitute," and other fancy names more or less 
resembling turpentine, which are petroleum and paraffin pro- 
ducts rather heavier than naphtha. A sample of such examined 
by the author had a specific gravity of 0-7913, a flash point of 
36 C. (97 F.), and began to distil at 156 C., the rate of distilla- 
tion being as follows : 

25 per cent, below . . . . . 180 C. 

50 200 C. 

70 210 C. 

76 215 C. 

No further portion was distilled. 

Such products are rather slow in drying, but they are better 
substitutes than petroleum spirit, because they are less inflam- 
mable, and there is, therefore, less risk of fire in using them. 
They do not oxidise or leave any residue on evaporation. 

COAL-TAR NAPHTHA. Coal-tar naphtha is not much 
used in paint-making; but in the preparation of varnishes it finds 
some use. It is one of the products obtained during the distilla- 
tion of coal-tar. In this operation several products are obtained, 
the proportion and nature of which depends partly upon the com- 
position of the tar and partly upon the manner in which it is 
distilled. Among the products is a light oil or spirit of a dark 
brown colour, known as "naphtha "or "light oil;" in specific 
gravity it ranges from 0*840 to 0'940. Its odour is character- 
istic, but somewhat disagreeable. Its composition is complex, 
containing hydrocarbons of the paraffin and olefine series in small 
quantities, but its characteristic constituents are hydrocarbons 
of the benzene series, such as benzene, C 6 Hg; toluene, C* H 8 ; 
xylene, C 8 H 10 ; cumene, C 9 H 12 ; durene, C 10 H 14 ; besides these 
it contains ammonia, aniline, toluidine, and other nitrogenous 
bodies ; alcohol, phenol, acetic acid, and sulphur compounds. 

It is purified by re-distillation, when what is called " once run 
naphtha " is obtained ; this is further purified by treatment with 
sulphuric acid, which removes all the basic bodies, the hydro- 
carbons of the olefine and crotonylene series, and the higher 
members of the benzene series. After separating the acid from 



COAL-TAR NAPHTHA. 377 

the semi-purified naphtha, the latter is treated wtih caustic lime 
or caustic soda, which removes all the oxygen and sulphur com- 
pounds ; finally, the naphtha is well washed with water, and is 
then ready to be finally purified by a re-distillation. 

Once run naphtha has a specific gravity of 0-886 to 0*893, and 
is the raw material for the preparation of 90 per cent, benzol, 
" 50/90 per cent, benzol," 30 per cent, benzol, solvent naphtha, 
and burning naphtha, as the commercial products are named. 

The benzols are light products used in the manufacture of 
aniline dyes. Burning naphtha, which has a specific gravity of 
about 0'880 to 0-887, is sold for burning in out-door lamps, 
especially costermongers' lamps, although it has of late been 
largely displaced by the petroleum oils for this purpose. 

Solvent or coal-tar naphtha is largely used in the india-rubber 
industry, and for making varnish. It is a water-white liquid, 
having a peculiar and characteristic odour of coal-tar hydro- 
carbons ; in specific gravity it varies somewhat, the usual range 
being between 0-865 to 0-877. On being subjected to distilla- 
tion, it gives from 8 to 30 per cent, of distillate below 130C., 
while as a rule 90 per cent, distils over below 160 0. It 
burns with a very smoky flame, and is very inflammable, the 
flash point being about 120 F. It is miscible with alcohol, ether, 
turpentine, petroleum spirit, shale naphtha, and other similar 
solvents, while it is a good solvent for oils, fats, resins, and is 
almost the only solvent for coal-tar pitch, and other pitches. 

In com position it is very complex, but it consists chiefly of the 
three isomeric, para-, meta-, and ortho-xylenes, C 8 H 10 , cumenes, 
small quantities of paraffins and olefines, and occasionally traces 
of naphthalene. Sulphuric acid has little or no action on coal- 
tar naphtha, but nitric acid has a powerful action, and trans- 
forms the coal-tar hydrocarbons into the nitro derivatives, nitro- 
xylenes, C 8 H 9 NO 2 ; nitro-cumenes, &c. Hydrochloric acid, 
caustic soda, and caustic potash have no action on it. 

It is used in making cheap quick-drying varnishes, rosin being 
the usual substance added to give the requisite coat ; it is more 
volatile than turpentine, although it does not leave any residue 
behind it. 

Commercial coal-tar naphtha is occasionally adulterated with 
petroleum or shale spirit, or with petroleum or paraffin burning 
oils ; in every case the specific gravity and flash points are 
reduced. The addition of the petroleum and shale spirits causes 
it to distil at lower temperatures and a little more regularly, 
while the burning oils raise the distillation temperatures rather 
considerably. 



378 PAINT VEHICLES. 

Such additions may also be detected by treating the suspected 
sample with a well-cooled mixture of sulphuric and nitric acids, 
which converts all the coal-tar hydrocarbons into nitro-coin- 
pounds, while the paraffin or petroleum oils are unaffected ; if 
now water is added, the nitro bodies, being heavy, sink to the 
bottpm, while the petroleum hydrocarbons being light rise to the 
top, and may be collected and measured. It should be pointed 
out that finding a small amount of such unchanged hydrocarbons 
does not necessarily indicate adulteration, as coal-tar naphtha 
naturally contains small quantities of paraffin hydrocarbons. 

The best method of examining coal-tar naphtha for its quality 
is by distillation. The method commonly used is the fol- 
lowing: 100 cc. of the naphtha is measured by means of an 
accurate glass measure into a tubulated retort of 200 cc. capacity; 
through the tubulure is inserted a thermometer, the bulb of which 
reaches within f of an inch of the bottom of the retort. The 
beak of the retort is connected with a long Liebig's condenser, 
and the distillation carried on by means of a Bunsen burner. It 
is best to insert the bulb of the retort into a deep sand bath, so 
that if the retort should crack, the naphtha would flow into and 
be absorbed by the sand, and no disastrous results ensue. The 
temperature at which the first drop flows from the end of the 
condenser is noted; with naphtha this occurs at about 110 C. 
Then the rate of distillation is noted; at 120 C. about 20 per 
cent, will come over, at 130 C. about 60 per cent., at 140 C. 
about 72 per cent., and 90 per cent, usually comes over below 
150C. Or, instead of taking the temperatures, as in the above 
example, and noting the quantity distilled at them, the tempera- 
ture at which each successive 10 cc., or 10 per cent., comes over 
may be noted ; the results will then be somewhat as follows : 
10 per cent, at about 128 C., 20 per cent, at 130 C., 30 per cent, 
at 132 C., 40 per cent, at 135 C., 50 per cent, at 137 C., 60 per 
cent, at 140 C., 70 per cent, at 145 0., 80 per cent, at 148 C., 
90 per cent, at 158 C. Addition of petroleum or shale spirits 
increases the proportion distilled at the different temperatures, 
while petroleum or paraffin burning oils decreases the propor- 
tion considerably. Sometimes the makers take out the lower 
benzene hydrocarbons and thus reduce the value of the coal-tar 
naphtha for the particular purpose ; in such case, the tempera- 
ture of distillation will be increased. 

The following table shows, in a comparative form, the proper- 
ties of turpentine, rosin spirit, shale and petroleum spirits, and 
coal-tar naphtha : 



METHYLATED SPIRIT. 



379 





Turpentine. 


Kosin Spirit 


Shale and 
Petroleum Spirits. 


Coal-tar 
Naphtha. 


Smell, cold, . 


Peculiar. 


Turpentinous. 


None or slight. 


/Smell of 
\ coal-tar. 


,, on warm- 
ing, . 





Of rosin. 


Slight. 


/Smell of 
\ coal-tar. 


Taste, . . . 


Characteristic 


( Peculiar 
| after-taste. 


Variable 
and slight. 


j- Strong. 


Specific gravity, 
Fluorescence, . 


0-867 
None. 


0-883 
Slight. 


0-730-0-760 
Slight. 


0-870-0-877 
None. 


Rotary power, 


Strong. 
Slight; poly- ) 


tt 


None. 


55 


Sulphuric acid 


merises on > 


it 


Slight. 


Slight. 




heating. ) 








Nitric acid, . . 


Strong. 


J J 


5 > 


Strong. 


Boiling point, . 


156 C. 


( High and 
( variable. 


Low and 
variable. 


High and 
variable. 


Flash point, | 


36 C. 
97 F. 


37 C. 
98 -5 F. 


Ordinary 
temperature. 


40 C. 
104 F. 



METHYLATED SPIRIT Methylated spirit is a very 
useful article in the preparation of varnishes and enamel paints. 
It consists essentially of a mixture of two bodies, methyl alcohol 
and ethyl alcohol; but in the ordinary commercial qualities there 
are usually small traces of other bodies, some of an ethereal 
character, others of an acid character. 

The alcohols are a very large and important group of chemical 
compounds, many of them finding extensive application in the 
various chemical arts. The type of the group is ethyl alcohol, 
C 2 H 5 O H, the body usually understood by the term alcohol when 
used by itself. It is also known as spirit of wine, for to it is 
due the intoxicating effect of wines, spirits, beers, and all 
beverages which have undergone fermentation. 

Pure ethyl alcohol is a colourless, very limpid liquid, having a 
pleasant odour and a hot burning taste. It is very volatile when 
exposed to the air, passing off completely and leaving no residue 
behind. It boils at 78'5 C. (173 F.) and distils over completely 
and unchanged at that temperature. It is only solidified when 
subjected to the very low temperature of- 130 C. The specific 
gravity of pure alcohol at 15-5 C. (60 F.) is 0'7935 ; but it has 
such an affinity for water that the preparation of a sample 
absolutely free from water is exceedingly difficult, so that the 
gravity given above may not be quite correct, but the error, if 
there is any, is small. Alcohol mixes with water in all propor- 
tions ; if the two bodies are fairly pure the proportion of alcohol 



380 PAINT VEHICLES. 

may be ascertained by simply determining the specific gravity 
(see table on p. 382). It mixes with ether, chloroform, tur- 
pentine, carbon bisulphide, and benzol, but not with petroleum 
products. It dissolves fatty acids and castor oil readily, but it 
has only a slight solvent action on the other fatty oils. It 
dissolves rosin and a few other resins, such as, shellac, sandarac, 
mastic, more or less completely ; but it will not dissolve the hard 
copals, animi and kauri. It is a powerful solvent for coal-tar 
dyes, and other bodies. 

When subjected to the action of oxidising agents it is first 
transformed into aldehyde, C H 3 O H, and then, finally, into 
acetic acid, C H 3 C O O H. 

It is obtained as a product of the fermentation of sugar ; this 
body, which is present in grapes, malt, and fruits of all kinds, if 
kept under conditions which cause it to enter into fermentation, 
loses carbonic acid and water, while alcohol is formed in fair 
proportions ; this passes into the water in which the process is 
conducted and from which it is separated by distillation, and 
redistillation with the aid of quicklime. 

The alcohol ordinarily met with in commerce is known as 
"rectified spirit of v/ine; " this has a specific gravity of 0-838, 
and contains 86 per cent, of real alcohol ; what is known as 
"proof spirit" has a specific gravity of 0'926, and contains 49 per 
cent, of real alcohol. 

Alcohol alone is not used in the preparation of varnishes as 
the high rate of duty levied by the Excise Authorities prohibits 
its use for this purpose. 

Methyl alcohol is a homologue of ethyl alcohol ; and has the 
composition indicated by the formula C H 3 O H. When pure it is 
a colourless liquid, very mobile and volatile, which has a fragrant 
spirituous odour, and boils at 55 C. Its specific gravity at 15'5 
C. (60 F.) is 0-8021, but authorities vary a little on this point. 
It is miscible in all proportions with water, from which it is not 
easily separated ; it also mixes freely with alcohol, ether, tur- 
pentine, &c., and possesses great solvent properties for resins, &c. 

When subjected to the action of oxidising agents it is first 
changed into formaldehyde, H C O H, then into formic acid, 
H C O O H. 

Methyl alcohol is obtained in large quantity in the dry 
distillation of wood. The wood is placed in iron stills or retorts 
in suitable furnaces, when there come over gaseous vapours, 
which condense, partly into an aqueous layer, and partly into a 
tarry mass. The aqueous layer, which has an exceedingly com- 
plex composition, contains acid, alcoholic, phenolic, ethereal and 



METHYLATED SPIRIT. 381 

other compounds. It is separated from the tar, treated with 
slaked lime, and then subjected to heat; crude wood-spirit distils 
over, while impure acetate of lime is left behind in the still. 

The spirit is very impure, and is further treated by redistilling 
for quicklime, then treating with sulphuric acid (which removes 
ammonia and methylamine) and, finally, redistilled with lime. 

Crude wood- spirit, as obtained by the above process, is a liquid 
of complex composition, containing about 95 per cent, of methyl 
alcohol in the best qualities, although some samples do not 
contain more than 40 or 50 per cent. The following bodies are 
found in wood-spirit, or wood-naphtha as it is sometimes called: 
Methyl alcohol, CH 3 OH; acetone (C H 3 ) 2 C O, sp. gr. 0'792, 
b. p. 56-5 C.; allyl alcohol, C 3 H 5 O H, sp. gr. 0-8604, b. p. 
96-5 C.; furfurol, ketones, &c. 

The odour of wood-naphtha is characteristic and somewhat 
unpleasant. It is due entirely to the impurities which are 
present in the spirit. Its taste, for the same reason, is ex- 
tremely nauseous; hence the use of wood-naphtha in making 
methylated spirit. 

Wood-naphtha is used for dissolving gums and resins in 
varnish making, and it is worth noting that many of the gums 
are more freely soluble in the crude wood-naphtha than they are 
in the pure methyl alcohol. The cause of this increased solvent 
power of the crude spirit must reside in the ethereal impurities 
it contains, many of which dissolve resins more freely than does 
methyl alcohol. 

The following reactions serve to distinguish wood-spirit from 
pure methyl alcohol: caustic soda gives a brown colour, sul- 
phuric acid a red colour, which increases in depth on heating; 
mercurous nitrate gives a grey precipitate of mercury. 

Methylated spirit is a mixture of 90 parts of rectified spirit 
of wine with 10 parts of wood-spirit, and this mixture is permitted 
by the Excise authorities to be sold, under special regulations, 
for manufacturing purposes free of duty, the addition of the 
wood-spirit rendering the spirit undrinkable. Of late, however, 
owing to improvements in the manufacture of the wood-naphtha, 
much of the nauseous taste is removed, and the methylated 
spirit now made is not so undrinkable. On this account the 
Excise authorities have recently compelled the addition of per 
cent, of petroleum oil to the methylated spirit, with the object 
of rendering it still more undrinkable, but the use of the original 
spirit is still by special permit allowed. 

The methylated spirit is usually sold at a strength of " 64 over 
proof," and has a specific gravity of 0-82^ Ifc-eentaiiis 90 per 




382 



PAINT VEHICLES. 



cent, of real alcohol. The meaning of the term "64 over proof" 
is that when 100 volumes of this spirit is mixed with 64 volumes 
of water, there is obtained "proof spirit," which is a spirit of 
such a strength that when mixed with gunpowder it will not set 
fire to the powder when a light is put to it. The term " proof 
spirit " is very vague, and should be done away with. It would 
be better to sell the spirit according to the actual quantity of 
alcohol it contains. 

The strength of methylated spirit may be fairly accurately 
estimated from its specific gravity. Tables have been constructed 
showing the quantity of alcohol contained in spirit of different 
gravities. Space cannot be spared in this book for the reproduc- 
tion of those tables, but the following table contains some infor- 
mation on this point which may be of use : 



Sp. Gr. at 
60 F. 


Per Cent, of 
Alcohol. 


Per Cent, of 
Proof Spirit. 


Sp. Gr. at 
60 F. 


Per Cent, of 
Alcohol. 


Per Cent, of 
Proof Spirit. 


079384 


100 


175-25 


0-880 


66-7 


129-5 


0-800 


98 


173 


0-890 


62-3 


122-5 


0-810 


94-6 


169-2 


0-900 


58-0 


115-3 


0-820 


91 


164-75 


0-910 


53-57 


107-6 


0-830 


87-2 


159-75 


0-91984f 


49-24 


100 


0-8382* 


84 


155-45 


0-920 


49-16 


99-8 


0-840 


83-3 


154-5 


0-930 


46-64 


91-64 


0-850 


79-3 


148-8 


0-945 


39-8 


85-59 


0-860 


75-1 


142-6 


0-950 


34-5 


82 


0-870 


70-84 


136 









Rectified spirit of wine. 



t Proof spirit. 



Methylated spirit generally has an acid reaction, due to the 
presence of small quantities of acetic acid and aldehyde; besides 
these, it contains traces of higher alcohols (amyl alcohol, propyl 
alcohol), oily and resinous bodies, ethereal compounds, and water. 

Methylated spirit is used in making varnishes from shellac, 
sandarac, rosin, mastic, dammar, and other resins ; such var- 
nishes are very quick in drying owing to the volatility of the 
methylated spirit. It is also used in the preparation of enamel 
paints. 

The quality of methylated spirit may be ascertained by distill- 
ing 100 c.c., when nearly all should be distilled below 100 C., 
the great bulk passing over between 80 and 90 C. The specific 
gravity is also a good indication of the quality, as shown in the 
table given above. In making any determination of the specific 



FINISH. 383 

gravity particular attention must be paid to the temperature at 
which it is determined, as small variations of temperature cause 
considerable alteration in the gravity; the standard temperature 
is 15-o C. (60 F.). The actual determination may be made by 
means of an hydrometer either the glass one ; or the metal 
one, known as Sikes' hydrometer, which is used by the Excise 
authorities ; or the specific gravity bottle may be used. 

Finish or methylated finish is methylated spirit containing 
about 3 oz. of rosin to the gallon. For some purposes this may be 
used in the place of methylated spirit, as the Excise do not place 
so many restrictions on its sale. It may be distinguished from 
the pure spirit by its giving a very copious white precipitate 
when water is added to it. 

On the Continent distilled animal or "Dippels" oil is used for 
the denaturing (or rendering undrinkable) of alcohol ; the use of 
this material has not been adopted in this country. 



384 



CHAPTER XIII. 
DRIERS. 

DRIERS are a class of bodies added to oil for the purpose of 
causing it to dry quicker than it would otherwise do. The 
bodies generally used for this purpose are salts of iron, lead, 
manganese, and zinc. The following list comprises all the com- 
pounds used as driers in paints and varnishes : Red lead, 
litharge, lead acetate, lead borate, manganese oxide, manganese 
sulphate, manganese borate, manganese oxalate, zinc oxide, 
zinc sulphate, and ferrous sulphate. 

Of these, the lead salts are most in use ; the manganese com- 
pounds are largely used ; the others but rarely. 

Red Lead is fully described on p. 93, et seq. 

Litharge is the monoxide of lead, and has the composition 
shown by the formula Pb O. It is prepared by oxidising lead 
in a current of air at a temperature sufficiently high to melt the 
oxide as it forms. On cooling, the litharge separates out in the 
form of flakes of a red-brown colour, which, on being ground up, 
forms a buff-coloured powder. Litharge is sold in the two forms 
here noted. It is soluble in dilute nitric acid and in acetic 
acid, forming the corresponding nitrate or acetate of lead. 
Hydrochloric acid dissolves it on boiling, forming the chloride; 
while sulphuric acid does not dissolve it, but forms the insoluble 
sulphate of lead. Mixed with oils, a slow action sets in, resulting 
in the formation of lead soaps, which are insoluble in water and 
many solvents. This action occurs with linseed oil \ the lead 
linoleate so formed dissolves in the rest of the oil, forming a 
kind of A r arnish, which, on drying, leaves a lustrous coat. It 
is this feature of lead salts that makes them valuable in the 
production of paint. Litharge is a powerful drier, and should 
not be used too extravagantly in the boiling of oil ; about J Ib. 
to the cwt. of oil is quite sufficient. 

Red lead is also a good drier, even better than litharge, from 
f Ib. to 1 Ib. being sufficient for 2 cwts. of oil. Its action on 
oil partakes more of the nature of an oxidising action than does 



MANGANESE DIOXIDE. 385 

that of litharge, while it is dissolved in the oil in the nature of 
a lead soap, as is the case with litharge. Both litharge and red 
lead are largely used in the preparation of boiled oil. The oil 
so prepared has a dark red colour, but dries quickly, and leaves 
a coat which is elastic and yet firm to the touch, so that it is 
capable of resisting a great deal of rough wear and tear, as also 
exposure to considerable variations of temperature. 

Lead acetate, Pb 2 C 2 H 3 O 2 , is a white crystalline solid pre- 
pared by dissolving lead or litharge in acetic acid, and evaporat- 
ing the solution down to dryness, or until it crystallises. It is 
readily soluble in water, and to a small extent in alcohol. It 
is used as a drier, principally for mixing with paints, and then 
it gives good results. Paint to which lead acetate, or sugar of 
lead as it is called, is added dries better on greasy surfaces than 
paint to which nothing has been added. As a drier it is not 
equal to either red lead or litharge, but it has the advantage 
of not causing the oil or paint to become dark or coloured. It 
is used in making what are called patent driers. 

Lead borate is a white powder prepared by adding a solution 
of borax to one of lead acetate or nitrate. The precipitate is 
collected, washed, and dried. It is largely used as a drier both 
in boiling oil and in mixed paints. It does not lead to the dis- 
colouration of the oil so much as red lead, while its drying 
properties are nearly equal to those of litharge. From J Ib. to 
1 Ib. is sufficient for 2 cwts. of oil or paint. 

Manganese dioxide, the black oxide of manganese, Mn 2 , 
is now very extensively used as a drier. It comes into the 
market from two sources, one natural, the other artificial. The 
natural manganese forms the mineral manganese or pyrolusite, 
and is found widely distributed in large quantities; for use, 
it is simply ground to a powder with water and then dried. 
It forms a greyish-black powder insoluble in water. Artificially, 
manganese is obtained from the still liquors of the bleaching- 
powder manufacturer, who, to prepare chlorine, treats manganese 
with hydrochloric acid, when he obtains a solution of manganese 
chloride, Mn C1 2 ; this is treated by a process invented by Wei- 
don, when all the manganese it contains is recovered in a usable 
form. While much of this recovered manganese is used over 
again in the preparation of chlorine, some of it is sold for other 
purposes. Manganese dioxide is soluble in hydrochloric acid 
with evolution of chlorine and the formation of manganese 
chloride, Mn Cl,, ; in sulphuric acid it dissolves with evolution 
of oxygen and the formation of manganese sulphate, Mn S O 4 . 
Essentially it is a peroxide, a class of bodies which may be 

25 



386 DRIERS. 

described as containing more oxygen than is exactly equivalent 
to the metal present in them ; this extra oxygen is often rather 
loosely combined, and ready to enter into combination with other 
bodies ; it is this feature in the composition of manganese which 
makes it useful in oil boiling, for the oxygen, during the process, 
combines with the oil and oxidises it, while the manganese dis- 
solves to some extent in the oil in the form of a manganese 
compound of the iinoleic acid of the oil. Manganese is in 
consequence a powerful drier; in fact, the most powerful known. 
The proportion usually added in the process of boiling is J Ib. 
to 1 cwt. of oil, and it is not desirable to increase this propor- 
tion much, as this would give rise to too much drying action, 
and cause the oil to form a hard and rather friable coat, not a 
firm elastic coat as it should do. Unfortunately, manganese 
has a tendency to make the oil dark. It is not a good drier for 
mixed paint, chiefly because the tendency in using it would be 
to add too much. 

Manganese sulphate, Mn S O 4 , is prepared by dissolving 
manganese in sulphuric acid, and evaporating the solution down 
to dryness. It is a crystalline salt of a faint pink colour and 
somewhat hygroscopic properties; hence, it should always be 
dried before using as a drier. Its drying action is, perhaps, 
rather more powerful than that of the lead compounds, but less 
than that of the last-named compound. Rather less than J Ib. 
should be added to each cwt. of oil or paint. It possesses one 
advantage over manganese in not adding to the colour of the oil. 
Owing, however, to its somewhat hygroscopic properties it is not 
largely used as a drier. 

Manganese borate is a powder of a faint pinkish hue prepared 
by adding a solution of borax to one of a manganese salt, such 
as the sulphate or acetate ; the powder is collected, washed, and 
dried, when it is ready for use. As a drier it is one of the best 
and most powerful, being superior to the lead compounds, but 
inferior to manganese, although it has the advantage of not 
leading to any discolouration of the oil. Between J and J Ib. 
is required for each cwt. of oil. 

The use of manganese linoleate, prepared by adding a solution 
of linseed oil soap to one of manganese chloride, has been lately 
proposed and patented. By using this product in a special way 
" boiled " oils are obtained much heavier than ordinary boiled 
oil, yet paler in colour. 

Manganese oxalate, Mn C 2 O 4 , has lately been proposed to 
be used as a drier for oil, and is said to have some advantages 
over other manganese compounds. It is prepared by precipitating 



ZUMATIC DRIER. 387 

manganese salts with oxalate of potash or soda ; or by treating 
manganese hydroxide with oxalic acid. One advantage is said 
to be that during the process of oil-boiling it is decomposed, and 
that, owing to the manganese dissolving in the oil in combination 
with the linoleic acid and to the oxalic acid being evolved in the 
form of carbonic acid, the metal is able to exert its greatest 
drying power. From J to i Ib. may be used per cwt. of oil. 

Zinc oxide, Zn O, is used as a drier, but cannot act as such as 
it has no drying properties at all. It is often put in mixed 
driers (see below), where it acts as a diluent to decrease the 
drying power of the other ingredients. For a description of its 
properties see p. 58. 

Zinc sulphate, Zn S 4 , is also used as a drier, but its virtues 
in this respect are rather problematical. As the commercial 
product contains much water of crystallisation it is necessary to 
dry it before it is added to the oil. 

Ferrous sulphate, copperas, Fe S O 4 , is frequently added as a 
drier, especially in the preparation of varnishes. It is in the 
form of pale bluish-green crystals, containing 5 molecules of water 
of crystallisation ; hence, before being used, it must be dried to 
dehydrate it. It is soluble in water. It is prone to decomposi- 
tion by oxidation, especially if the crystals be exposed to moist 
air; it is this property of being changed by oxidation from 
ferrous sulphate to ferric oxide that makes copperas useful as a 
drier. It should be used with care, as its tendency is to harden 
the coat of paint or varnish, and thus impart a tendency to crack. 
It is not by any means such a good drier as either lead or man- 
ganese salts ; from 1 to 2 Ibs. are required for 1 cwt. of oil. 

Besides the simple driers described above, a variety of com- 
pound driers, usually composed of mixtures of the single driers 
in various proportions or with some linseed oil or boiled oil, are 
made ; it is not intended to describe these in detail, but a few 
recipes for the production of those principally in use will be 
given. 

Patent Driers. Take 15 Ibs. of dried zinc sulphate, 4 Ibs. of 
lead acetate, and 7 Ibs. of litharge ; mix them with 4 Ibs. of 
boiled oil, and grind well together. Mix 100 Ibs. of Paris white 
and 50 Ibs. of white lead with 30 Ibs. of boiled oil, grind, and 
then mix with the first mixture, adding sufficient boiled oil to 
give the mass the consistency of soft dough. 

The composition of the commercial " patent driers " varies with 
different makers, but the above is a common form. 

Zumatic Drier. 25 Ibs. of zinc white and 1 Ib. of borate of 
manganese are ground together. The object of the zinc white is 



388 DRIERS. 

simply to dilute the manganese salt, and to form a powerful drier 
in a convenient form. The proportions generally used are 1 Ib. 
of the drier to 25 Ibs. of paint. 

Zinc Drier. 6J Ibs. of dry manganese sulphate, 6J Ibs. of 
dry manganese acetate, 6J Ibs. of dry zinc sulphate, and 980 Ibs. 
of zinc white are ground together. From 2 to 3 per cent, of this 
is usually added to the paint. This is called zinc drier, because 
it was brought out as a drier for zinc white. It is also known as 
Guynemer's drier. 

In both the above mixtures the manganese salts only act as 
driers ; the other materials are really diluents, and of themselves 
can exert no drying action. 

Oxidised Oil Driers. Oxidised oil or well boiled linseed oil 
makes a good drier, very useful in many cases. 



389 



CHAPTER XV. 
VARNISHES. 

VARNISHES form a very important group of the materials used by 
the painter in carrying on his art. They are liquid bodies, more 
or less coloured, although colour is not an essential feature. 
When applied to the surface of a body, they lose a portion of 
their constituents by evaporation, and there is left a coat of a 
highly lustrous and durable character, thereby increasing the 
lustre of the object, developing its beauty, and protecting it from 
the destructive action of the atmosphere. 

When varnishes were first introduced is very uncertain, but 
the kinds now in use are of modern origin, and'mostly of English 
introduction, English varnishes being superior to those of any 
other country. Their use has, during the present century, in- 
creased very considerably, and has now attained very large 
proportions. 

The subject divides itself into two parts 

1st, Varnish materials. 
2nd, Varnish making. 

1st, VARNISH MATERIALS. 

The materials used in the manufacture of varnishes can be 
divided into six groups : 

1st, Drying oils. 

2nd, Resins. 

3rd, Gums. - 

4th, Solvents. 

5th, Driers. 

6th, Colouring matters. 

In the trade, the second and third groups are generally classed 
together under one head " gums." 

/ 1st, DRYING OILS. These have already been considered 
{seejp. 335, et seq.\ and very little requires to be added here. 



390 VARNISHES. 

Linseed oil only has as yet been used in the preparation of 
oil varnishes, although some of the other drying oils could be 
used; but it is very doubtful whether they will give such good 
quality of varnish as linseed oil, and they are more costly, 
which is an important item in varnish making. The linseed 
oil used for making varnish should be of the very best quality 
the best Baltic; other varieties of linseed oil only yield poor 
qualities of varnish. It should be kept at least twelve months 
before being used. It is best stored in old steam boilers, and 
it is essential that the air should be excluded. Nothing is 
definitely known as to the character of the action that goes 
on in the oil during the time it is thus stored; but there is a 
wonderful difference between unstored and stored oil in their 
varnish-making properties, the latter giving much the better 
results. 

2nd, RESINS. This is by far the most important group of 
varnish materials, for on these bodies the lustre and lasting 
properties of the varnishes depend. They are all of natural 
origin, being exudations from various species of trees.* They 
are very numerous. Some are used almost exclusively for varnish 
making, others are also used for other purposes, while some resins 
are not used for varnish making, but find use in other directions. 

As a class, they are distinguished by being more or less hard, 
friable or brittle, lustrous, generally clear and transparent, al- 
though some are slightly opaque, insoluble in water, and soluble 
in alcohol, ether, benzol, and other solvents of a similar character, 
to a greater or less extent. In composition, they are very com- 
plex, being mixtures of bodies having acid properties. A few 
only of these bodies have been isolated and their characters 
definitely ascertained. In their ultimate composition they are 
rich in carbon, poor in oxygen, and contain no nitrogen. They 
are more or less combustible, usually burning with a smoky 
flame. They are usually devoid of colour, which is a valuable 
feature for varnish making, although some are coloured. As a 
rule, they are free from odour, but some possess fragrant and 
characteristic odours. 

* All exudations from trees which form hard, more or less brittle masses, 
are termed in the produce markets and trade generally " gums " e.g., gum 
arabic, gum tragacanth, gum copal, gum animi, gum sandarac, &c., no 
matter what their origin or properties. True gums are those which are 
soluble to a greater or less extent in water (see p. 414), and resins are 
bodies which are not soluble in water, but soluble only in solvents like 
alcohol, turpentine, &c. In the following pages the word "gum" will 
(except as a trade description) be used exclusively for gums proper, and 
" resin " for the true resins, of which copal and animi are examples. 



RESINS. 391 

Classification of Resins. The resins can be classed into 
various groups. Cooke classes them into three divisions : 1st, 
Resins ; 2nd, Gum-resins ; 3rd, Oleo-resins. The resins possess 
the properties enumerated above, and will be again referred to 
below. The gum-resins contain a little gum as well as resin in 
their composition ; very few are used for varnish making. The 
oleo-resins consist essentially of a mixture of resin with a liquid 
oil which imparts to them a viscid character; they are useful 
bodies, although few find their way into varnishes. The resins 
can be divided into two groups hard or copalline, soft or elemi 
resins; another method of grouping them is into oil- varnish 
resins, ethereal-varnish resins, and spirit-varnish resins. This 
latter classification will be adopted in this book as being of a 
practical nature. 

Characters of Resins. Some of these have been pointed out 
above in a general manner, but it is advisable to deal with them 
in a more detailed manner. The characters of resins which are 
of the most importance are form, appearance, colour, hardness, 
specific gravity, solubility. 

Form. Most resins occur in the form of knotty masses, some 
in the form of drops, and others in that of cylindrical pieces. The 
resin flows out of the tree in the form of drops ; if the resin 
solidifies quickly it keeps this form, as, for example, mastic; if 
the process of solidification is slow, then the resin tends to form 
into tears or cylindrical pieces, as, for example, sandarac; if, 
again, the process goes on slower and the resin collects on the 
tree or drops on to the ground, it forms into knotty masses 
of various sizes and shapes, as, for example, copal, animi, dam- 
mar, &c. Some resins come into the market artificially shaped, 
as, for instance, gamboge in cylinders, shellac in thin plates, 
dragon's blood in thin sticks or powder, benzoin and elemi in 
blocks. 

Appearance. The appearance of many of the resins is charac- 
teristic. Animi is clear and transparent, and has a peculiar rough 
surface which, from its appearance, is known as the goose skin. 
Benzoin, elemi, and some others have more or less an agglomerate 
appearance, as if made up of two or three kinds of gum ; such 
a structure is called by mineralogists amygdaloidal. Animi and 
copal have a very lustrous appearance ; elemi, benzoin, &c., are 
more waxy ; kauri partakes somewhat of both, and has a semi- 
lustrous appearance. Some animi, copal, sandarac, dammar, and 
kauri are more or less crystalline; while elemi and benzoin are 
amorphous in appearance. These points will be dealt with in 
detail when describing the resins individually. 



392 VARNISHES. 

" Colour. Resins vary very much in colour. Accroides or xan- 
thorrea is deep yellow, copal mostly of a pale straw, animi is a 
shade deeper, amber is of a brownish-yellow, and dragon's blood of 
a red colour; dammar is almost colourless; shellac has an orange 
colour; elemi is usually of a greyish tint, ammoniacum of a brown, 
and asphaltum a deep brown, almost approaching a black ; the 
large majority of the resins are of a pale brownish colour. The 
colour of resins is a most important feature in varnish making, 
inasmuch as any colour they may possess affects the colour of the 
varnish made from them ; the difference between two varnishes 
sometimes arises from the quality of the resin used, not in the 
kind of resin or other materials, nor in the method of preparation. 
In the best varnish the resin used is of the best quality, and has 
the purest colour; while inferior varnish is made with an inferior 
quality of resin, the colour of which is rather darker. In copal 
varnishes, for instance, where there are so many varieties of 
copal, so far as regards colour (ranging from almost colourless to 
pale brown), the palest copals only are used for pale copal var- 
nishes, the oak varnishes being made from the darker coloured 
copals. 

, Hardness and Fusibility. In these two features, hardness and 
fusibility, there is a wide range of difference between the various 
resins. Some, such as the hard copals, amber, animi, are very 
hard and difficult to melt ; these resins are the very best of the 
varnish resins, giving the finest and most durable kinds of var- 
nishes, which are, however, the most difficult to make. Some 
resins, such as elemi, are so soft as to easily bend between the 
fingers, and will melt or become softened by the heat of the 
hands ; such resins do not give durable varnishes, and they are 
only used to tone down the hardness and brittleness of other 
resins. Other resins, as animi and amber, are hard and difficult 
to break up ; others again, as accroides, rosin, &c., are brittle 
and easily reduced to a powder ; the former class yield durable 
varnishes, which, when applied to a surface, will stand a good 
deal of wear and tear ; the latter class make varnishes which do 
not stand much wear and tear. The resins vary in the appear- 
ance of the fracture when broken; some exhibit a conchoidal 
or shell-like fracture, others a ragged appearance; some a powdery 
fracture, some a smooth fracture. The particular kind of fracture 
of each resin will be described when each resin is described in 
detail. 

Specific Gravity. Resins are all heavier than water; some, as 
copal, amber, and sandarac, are only slightly heavier; others, as 
benzoin, guaiacum, shellac, much heavier; the specific gravity of 



OIL-VARNISH RESINS. 393 

each particular resin does not vary very much, so that this is a 
distinguishing feature which may be taken advantage of in 
making an examination of resins. The following table gives the 
specific gravity of most of the varnish resins : 

Animi, 1 '043 to 1 "067. Thus, 1 '042. 

Amber, T074 to 1 094. Dragon's blood, T200. 

Benzoin, Siam, 1'235. Elemi, 1-019. 

Penang, 1-145 to 1-155. Gamboge, 1'025. 

Borneo, T165 to 1'170. Guaiacum, 1-236 to 1'237. 

Copal, Sierra Leone, 1'054. Kauri, 1'050. 

Angola, 1-064. Mastic, 1 '056 to 1 -06. 

Pebble, 1-055. Sandarac, 1 "038 to 1 '044, 

Manila, 1063. Shellac, I'll 3 to 1 -214. 

Dammar, 1'055 (Batavian). Accroides, M97. 

1-062 to 1-123 Locust, British Guiana, 1-030. 

(Singapore). Rosin, T044 to I'lOO. 

^ Solubility. This is a very important property of resins, and 
influences to a very great extent the use to which they are put. 
There is a great deal of difference between the resins in regard to 
their solubility in various media; some, as animi and copal, are 
insoluble in any solvent, and before they can be made into 
varnishes have to be fused by heat ; others, as shellac, sandarac, 
Manila copal, and rosin, are soluble in alcohol, ether, benzol, and 
are easily made into varnishes; other resins are not so easily 
soluble, such as dammar, mastic, &c. Acetone, ether, and benzol 
are the best solvents for resins; they will act more or less on all 
the resins of the last two classes. Petroleum spirit is a poor 
solvent for the resins, only rosin and dammar being dissolved 
by it. 

OIL- VARNISH RESINS. This class of resins includes 

Amber. Sandarac. 

Animi. Rosin. 

Copals. Kauri. 

As a class these resins are distinguished by being insoluble in 
oils, ether, and other solvents until after they have been more or 
less liquefied by fusion. There are one or two exceptions to this 
general statement; resin is soluble in alcohol as are also some of 
the soft copals and sandarac; but, otherwise, it holds good. 

Amber. This resin yields the finest and most durable 
varnish; its cost, however, prevents it from being very largely 
used for this purpose. Amber is a fossil resin found in the 
greensand beds of the Cretaceous period, in a narrow belt of 
districts extending from England through Holland, the Baltic 



394: VARNISHES. 

coasts of Germany, Russia, South Siberia, to North America; 
the quantity found in this country is unimportant, the great 
bulk of the supply coming from the Baltic coasts of East Prussia. 
The tree or trees which yielded amber are not known. Some 
authorities consider that the bulk of it was produced by a tree 
belonging to the Coniferce and named Pinites succinifer ; but, in 
the absence of definite information, this is simply speculation. 
Much of the European supply is washed up by the waves from 
deposits which are now under the sea. In West Prussia, how- 
ever, there are some deposits inland of great extent and these 
are worked by a regular system of mining, which has a great 
resemblance to gold mining in its methods. In the district of 
Samland, containing the towns Wansen, Grosskuhren, Hubnicken, 
Palmnicken, Kraxtepellen, Krieslacken, and a few other places, 
the amber deposits are found in a kind of blue earth, at a depth 
of about 108 feet from the surface and about 46 feet below the level 
of the sea; the thickness of the bed is from 8 to 28 feet, of which 
the lowest portions only are worked. The mining operations 
are carried on by means of shafts and levels, much as coal mining 
is done in this country; great precautions, however, have to be 
taken to prevent inrushes of seasand and water, the ground above 
being very loose and treacherous. The blue earth is taken to 
the surface and is there washed in a long slightly inclined trough, 
the entrance end of which is covered with a grating having 
apertures about 3 inches in diameter. The large masses are 
carefully broken up by hand; at intervals of about 6 feet men 
are stationed with nets to catch all the pieces of amber as they 
pass down the trough; finally, the waste or taillings pass from 
the trough through a sieve having apertures of about one-third of 
an inch in diameter which, while allowing the earth to pass 
through, retains the amber. At some places jigging machines 
with sieves having a mesh of about one-eighth of an inch are 
used with satisfactory results. About 12,500 tons of earth per 
month are treated and yields about 156 cwt. of amber at an 
average cost of about 4s. per Ib. 

On the coast amber is obtained from deposits under the sea by 
means of nets, diving and dredging ; in the latter case the 
dredged material is washed as above described. 

Amber is found in many other localities, but to a much smaller 
extent ; it is often washed ashore on the coasts of Norfolk, 
Suffolk, Essex, and Sussex, but the quantity is too precarious to 
make amber-collecting a profitable industry. It is also found in 
some sandy deposits in several London localities, but it is scarce. 
In several localities in France it is found in small quantities. 



OIL-VARNISH RESINS. 395 

Among the mountains of Roumania amber is found in workable 
amounts and of a fair quality. In America there are many de- 
posits, some of which are worked, while there are doubtless many 
others not discovered. In one particular instance (the amber 
from Vincent-town, New Jersey) the specific gravity is less than 
that of water. In North Burmah, in the Hukong valley, amber 
is found at an elevation of 1,050 feet above the sea and is regularly 
mined for. 

Amber is found in commerce in pieces varying much in size, 
form, colour, &c. That which is got from mines is usually 
angular ; sea amber is more or less rounded and pebble shaped. 
It varies in colour very much, from a dull white through pale 
yellow, to brown, blue, green, and red. In commerce amber is 
classified according to size, the largest pieces being the most 
valuable. The palest in colour are best and are mostly used for 
ornaments, tfec., while the darkest, which are of the least value, 
are chiefly used for varnish making. The following varieties are 
recognised in the trade : 1. "Shining;" pale yellow or greenish, 
very bright. 2. " Bastard," opaque, darker in colour. 3. " Bone 
colour ; " dull white ; this variety is usually very rich in succinic 
acid, and, hence, is mostly used for preparing that body. 4. "Agate 
colour." 5. " Impure ; " it contains remains of flies and other 
insects, vegetable matter, tfec. 6. " Cloudy ; " this is rather 
unequally coloured, but is mostly light yellow. 7. "Transparent ; " 
it has various colours. The value of amber depends on the size 
of the pieces, colour, transparency, &c. For varnish making, as 
a rule, only the small dark coloured pieces are used, as these are 
the cheapest. 

Amber has a specific gravity of 1 '074 to 1*094; it is insoluble 
in water, ammonia, carbon bisulphide, petroleum spirit, benzene, 
and acetic acid ; slightly soluble in absolute alcohol, turpentine, 
ether, chloroform, and some essential oils. Boiled in either 
linseed oil or rape oil for some hours it is softened and can then 
be moulded, but it is not dissolved. It has a high melting point, 
about 310 to 320 C. (600 to 615 R). When distilled it yields 
succinic acid, amber oil, and a solid residue. The amber oil has 
a more or less turpentinous composition, a specific gravity of 
0-9606 and forms about 23 per cent, of the resin. The melted 
resin is soluble in hot linseed oil, chloroform, ether, benzene, 
petroleum spirit, and turpentine, but is insoluble in alcohol. 
When heated just to the point of fusion for a short time the 
amber is altered and becomes soluble in turpentine, oils, and 
other solvents ; this property is taken advantage of in varnish 
making. Amber yields the best varnish of any of the resins ; 



396 VARNISHES. 

it gives a firm yet elastic coat and one which resists the action of 
the atmosphere exceedingly well, but as it is rather dark and 
expensive it is rarely used. 

Animi. Animi is the varnish resin par excellence. No other 
resin at all approaches it for the brilliance and durability of the 
varnish it will make, its nearest competitors being the copals. 
It is used in making the best quality of coach varnishes. 

Animi is imported into England from Zanzibar, although some 
inferior resin sometimes finds its way through Aden, Bombay, and 
other Indian ports. 

Animi is found in two forms "fossil," which is by far the 
best, and "recent" or "virgin," which is used locally and in 
India for making inferior varnishes. Both these forms are derived 
from trees, the specific tree which yielded the fossil resin is not 
known, but it is possible that it may be the same as now yields 
the recent resin. 

" Fossil " animi, known to the natives who procure it as " san- 
darusi," is found in many localities, but especially in the coast 
districts of the island of Zanzibar and on the neighbouring main- 
land. It occurs in patches only at a depth varying from \\ to 3 
feet below the surface, generally in a red soil and covered with 
decayed vegetation. Although found in soils of other colours 
than red, yet the Arabs who collect it aver that the red soil resin 
is by far the best and they rarely search for it in other places. 

The resin is only searched for during the rainy season, which 
extends from April to October, the ground then being sufficiently 
soft to permit of its being probed with a sharp-pointed rod. 
During the dry season, which occupies the remainder of the year, 
the ground is too hard to permit of this being done. The method 
of working is comparatively simple. The resin collector takes 
the iron rod and probes with it in what he may consider by expe- 
rience to be a likely spot, and the presence of the resin is readily 
ascertained ; then a hole of about 6 inches in diameter is dug 
and the resin taken out. The resin thus collected is stored until 
the end of the rainy season, when it is transported to Zanzibar 
and there sold to the resin merchants. The amount collected is 
comparatively small, generally averaging 1 Ib. per day for each 
man engaged, although it would be quite possible to collect 
more. 

The resin as it is found is covered with a red crust formed by 
the decomposition of the resin and more or less amalgamation 
with the red soil; this is removed when the resin reaches the 
resin merchants, usually by a method of scraping and treating 
with an alkali, when a peculiarity of gum animi makes its appear- 



OIL-VARNISH RESINS. 397 

ance that is, the so-called goose skin. The surface of the resin is 
found to be more or less covered with a regular pattern of inden- 
tations closely resembling the skin of a goose when it has been 
plucked; whence its name. This appearance is caused by the 
impression of the cellular structure of the tree in which the resin 
was originally deposited. The merchants at Zanzibar sort the 
resin into various qualities before they export it to this country. 

" Recent " or " virgin " animi is collected direct from the trees 
or picked up on the surface of the ground immediately under 
those trees. The tree which yields this recent resin is the Tracliy- 
lobium mossambicense, although this is a matter of doubt. The 
Arab name for the tree is "shajar el sandarus," but different 
tribes give it different names. This variety of animi has a pale 
colour, a very smooth surface, and, usually, a long tear shape. 
The tree is a most prolific producer, every part seeming to be 
so charged with resin, that the wounding of any part of the tree 
immediately causes a flow of and a deposit of the resin in that 
part, a fact of which the Arabs of the districts where it grows are 
not slow to take advantage. The resin is called by them " chakazi " ; 
it is soft, not unlike gum thus in consistency, of a dull appearance 
and of small value ; it is imported to India for making poor var- 
nishes. 

Animi occurs in homogeneous pieces of various sizes, although 
none are very large ; it is quite transparent and of a yellowish or 
brownish-yellow colour ; sometimes it contains the remains of 
insects. It is so hard that it cannot be scratched with the nail, 
and breaks with a more or less conchoidal fracture. The specific 
gravity of animi is about 1*062 to 1-068. 

It is nearly insoluble in all ordinary solvents, which at the 
most^ cause it to swell a little ; ether dissolves part, petroleum 
ether causes it to swell to a white mass, alcohol has a similar 
action, chloroform partially dissolves it. With any solvent but 
a small part dissolves. On this point authorities differ much, 
probably from the fact that they were not using a genuine 
sample of animi. Animi is not affected by boiling with caustic 
soda or acids. The properties here given refer to the fossil 
variety. 

When animi is distilled it gives off, at first, dark brownish 
vapours, but when the resin is quite melted, which occurs at a 
temperature of from 240 0. to 250 C. (450 to 465 F.), no appre- 
ciable vapours are given off; the rosin loses from 20 to 25 per cent, 
of its weight, of which about 2 per cent, comes off as water and 
18 -5 per cent, as a brownish coloured oil, having a strong empy- 
reumatic odour; the oil has a specific gravity of 0*9081, and is 



398 VARNISHES. 

soluble in benzene, ether, petroleum spirit, and turpentine, but is 
insoluble in alcohol. It has slightly acid properties and contains 
a, small quantity of bodies capable of combining with caustic soda, 
but the main bulk has, probably, a terpene composition. The 
residue in the retort sets, on cooling, to a hard, clear, transparent 
resin, soluble in benzene, chloroform, petroleum spirit, and tur- 
pentine to clear solutions ; in ether it gives a cloudy solution ; it 
is insoluble in alcohol, but soluble in hot linseed oil. 

Copal. The term "copal" is now used very frequently as 
a generic term, covering a number of resins of various origin. 
Strictly speaking, it should, when used as a class name, only 
include resins, of which animi may be taken as a type ; but it is 
made to include resins such as Manila copal, which has very 
different properties. Accordingly, copals are often divided into 
two groups "true copals" and "false copals." The former in- 
cludes animi and the copals found on the West Coast of Africa, 
and perhaps also kauri and the Demerara copal. The latter group 
includes Manila copal, dammar, and a few others. "True copals" 
are hard, lustrous resins, insoluble in solvents, and require to be 
fused before they can be made into varnish. The " false copals " 
vary very much in their properties some are hard, as dammar, 
others are soft, as Manila copal. They are more or less soluble in 
solvents like alcohol, benzol, turpentine, &c. 

Gum copal is a product of vegetable origin found on the 
West Coast of Africa, over a district extending from latitude 8 N. 
to latitude 4 S., a distance of some 700 miles. In this 
district very large quantities of copal is found, as a fossil 
resin, in the ground at various depths up to about 10 feet. It is 
dug for by the natives during the rainy season only, which 
extends from March to May. Generally it is found in the super- 
ficial strata of marl, clay, and sand, which are sufficiently soft in 
the wet season to permit of being easily worked, but in the dry 
season they are too hard for the negroes to dig into with the 
rather primitive tools they use. The botanical origin of copal 
is not known with certainty. Some authorities assign it to a 
tree called Guibourtia copalUfera, while it is quite possible that it 
comes from a variety of species; certainly, no tree which is at 
present growing on the copal coast yields this gum. This fact, 
together with its occurrence in very recent deposits and the 
existence of the pebble copals in the beds of the rivers of this 
district, show that it is most likely to be the produce of various 
species of trees which have grown or now grow in inland dis- 
tricts. Probably, as the interior of the copal districts become 
better known, the source of the resin will be discovered. After 



OIL-VARNISH RESINS. 399 

the resin has been collected, it is carried by the natives, along 
with other produce which they collect at the same time, to the 
ports for export to Europe, America, &c. The principal ports 
from which copal is thus exported are Sierra Leone (from which 
the best copal is obtained), Accra, Benin, Gaboon, Loango, the 
Congo, Angola, Benguela. The copals obtained from these places 
are not identical in properties, some being harder than others. 

Sierra Leone copal is the best quality of copal imported from 
Africa. It usually comes over in the form of rough angular 
pieces, almost colourless or, at the most, having a faint yellow 
colour. It is hard, has a specific gravity of 1-054, is quite in- 
soluble in all the ordinary solvents, and only becomes soluble 
after it has been fused, the melting point being 400 F. It is 
very lustrous, and makes a first-class and very durable varnish. 
By careful selection of the resin used, pale varnishes can be made 
from this copal. It is the product of the copal tree Copalifera 
Guibourtiana (Guibourtia copallifera), from which much of it 
is collected at the end of March, before the rainy season sets 
in. 

Pebble copal also comes from Sierra Leone. It occurs in 
the form of rounded pieces, varying slightly in size, and is mostly 
colourless; but some samples are slightly coloured, mostly pale 
brownish-yellow. It is very hard (perhaps the hardest of the 
copals), and its specific gravity is about 1-055. From its form it 
has evidently been transported from the interior by the rivers, in 
whose beds it is now found. Owing to the supply being small 
and somewhat uncertain, varnish makers are rather shy of using 
Pebble copal, although it makes a good varnish resin. 

Angola copal is found in the three districts of Angola, Ben- 
guela, and Congo, the copal from which are so much alike that in 
commerce they are classed together as " Angola copal," or, as it 
is often called from its colour, " red Angola copal." It is generally 
in the form of globular pieces, although flat pieces are often met 
with. The size is usually about 1 to 2 inches, but large pieces 
about 5 to 6 inches have been found. This copal is generally 
covered with a reddish crust about one-sixteenth of an inch 
thick. When this crust is removed, the resin is usually found 
free from colour or, at most, has a faint yellowish colour; 
occasionally brownish pieces are found. The darker coloured 
pieces are often the freest from air bubbles bits of bark, wood, 
&c., which are often found in the colourless variety. These, 
therefore, command a higher price in the market. Angola copal 
is not so hard as those above described. It yields a good quality 
of varnish, brilliant and durable. Its specific gravity is 1 -065. 



400 VARNISHES. 

Angola copal only comes into commerce in small quantities; 
almost too small to induce varnish makers to use it. 

Gaboon copal is found in rounded, flattened pieces, varying 
in size from about J to 2 J inches ; in colour it is the darkest of the 
copals a sherry colour; transparent; but, as a rule, not homo- 
geneous. The surface is generally smooth, but some pieces have 
a crust, which is sometimes striated. 

Loango copal differs from other copals in being found in the 
form of cylindrical pieces, which are evidently only portions 
broken off much larger pieces; two varieties are known, red and 
white; the former is, as its name indicates, of a reddish or faint 
brownish colour, and is rather larger than the latter and more 
cylindrical in form; the white varies in colour from colourless to 
faint yellow. The red is the best and, on account of its being 
more homogeneous, more lustrous and harder than the white, is 
both more in demand and brings a higher price in the market.* 

Demerara Copal. This fossil resin comes from the locust 
tree, Hymencea Courbarii, of British Guiana. It is collected much 
in the same manner as animi and copal, and, when freed from the 
outer crust, has a splendid lustrous appearance, being quite clear 
and transparent, perfectly homogeneous and often occurring in 
large masses; pieces of from 10 to 12 Ibs. weight are frequently 
found, while a mass weighing at least 60 Ibs. was shown at the 
Manchester Exhibition in 1887. 

It has a specific gravity of 1-030 and melts at about 240 to 
250 C. (450 to 465 F.); it gives off a large proportion of volatile 
oil and gas, and leaves but little solid resinous matter as compared 
with animi ; thus, weight for weight, it does not make as strong 
a varnish as animi or copal, but, allowance being made for this 
fact, it is an excellent varnish resin. It is also known as Deme- 
rara animi. 

Ether dissolves it, alcohol causes it to swell to a white jelly, 
chloroform causes it to swell and dissolves a part of it, petroleum 
ether causes it to swell to a white jelly. 

Kauri. This is a very important copal-like resin found in 
New Zealand in large quantities, which has only comparatively 
recently come into use as a varnish resin. It is also known as 
Cowdee gum, Kowree, Cawree gum ; there are various other ways 
of spelling the name. 

This resin is the product of various species of New Zealand 
pine trees, chiefly from Dammar a Australia, which yields by far 

* Copal is slightly soluble in turpentine on long digestion with the solvent, 
but it is nearly insoluble in other media, which at the most only cause it to 
swell, while but a small portion passes into solution. 



OIL-VARNISH RESINS. 401 

the larger proportion of the resin which finds its way into the 
market. Other species are Dammara ovata and D. Cookii of 
New Zealand, while in Queensland, D. Brownii, and in New 
Caledonia, D. lanceolata, yield the resin in small quantities. 
Most of the resin which finds its way into use is dug out of the 
ground. It is found all over the Colony. At present there is 
no regular organised system of searching for the resin, but a 
very large, although variable, number of persons, mostly natives, 
are engaged in the collection of it. The resin is found at various 
depths, from a few inches to several feet below the surface, on 
which, however, there are no indications of its presence below; 
large pieces are often turned up in ploughing cultivated lands. 
The resin digger uses two tools, a spear, which is a long, sharp- 
pointed, wooden-handled steel rod of about J inch in diameter, 
and a spade. With the spear he pierces the ground and feels for 
the resin; experience soon tells him when he has touched a piece, 
and then he digs down to it; the find may only be a small one, 
or, as occasionally happens, very large pieces are found. The 
surface of the resin is covered with a thick crust of decayed 
vegetable and resinous matter which is removed by the digger 
before he sells it to the merchant. Besides the fossil' resin, kauri 
is also obtained from the living trees; such resin is known as 
" young " kauri, and it differs from fossil kauri in being almost 
colourless and softer. The trees, when they are cut, bleed 
profusely and yield a pale yellowish viscid fluid which dries into 
a hard resinous mass ; both old and young trees yield the resin, 
and it is no uncommon thing to find in old trees deposits of the 
resin. The stumps of felled trees soon become covered with a 
thick layer of kauri resin. The trees are fast disappearing, 
and soon no " young " resin will be obtainable. 

Kauri comes into the market in pieces varying much in size, 
from a few inches to two feet, and in weight, from 1 Ib. to cwt. 
The best quality is known as "dial" kauri, but many varieties 
are recognised in the trade. In colour it varies but little, 
being mostly of a pale amber or pale brown. Some samples 
are homogeneous, others are more or less streaked ; the former 
kind are usually clear and transparent, while the latter are often 
opaque. In lustre it varies from glassy to opaline. It breaks 
with a conchoidal fracture, and freshly-broken pieces have a 
fragrant odour which is very characteristic. The taste is pleasant 
and aromatic. It is used by the Maoris of New Zealand as a 
chewing gum, especially the " young " kauri. The specific gravity 
is 1-050, New Caledonia kauri being rather heavier than New 
Zealand kauri. It melts easily at from 360 F. to 450 F. ; and 

26 



402 VARNISHES. 

on distillation it gives about 1'4 per cent, of water and from 
16 to 20 per cent, of oil of specific gravity 0-9224, soluble in ether, 
but insoluble in alcohol, and caustic soda. 

Kauri is used in varnish making as a rival to copal, but, 
although the varnish it yields is a good one, yet it is far from 
equalling copal varnish, not being so durable. When exposed 
to the weather it has a tendency to become powdery, but, being 
hard, it is largely used in varnishes for interior work and, to a 
limited extent, in the preparation of ornaments. Petroleum 
spirit, alcohol, turpentine, and benzol cause kauri to swell up to a 
white opaque mass dissolving a portion of it ; chloroform only 
partially dissolves it, but ether completely. After melting, the 
gum is soluble in ether, petroleum spirit, turpentine, benzol, and 
chloroform to clear solutions. There are, however, variations 
between different samples of kauri in their solubility in these 
media. 

Sandarac. This resin has quite different properties to the 
copals above described. It is used in making both oil and spirit 
varnishes, although for the former kind its use is decreasing. 
It is also known as gum juniper. It is the produce of the Alerce 
tree, Callitris quadrivalvis, a tree indigenous to North Africa, in 
the mountain regions ranging from the Atlantic coast to at least 
as far East as Eastern Algeria, but possibly further. The resin 
is found exuding naturally from the trees. The Moors who 
collect it are in the habit of making small incisions in the bark 
of the tree for the purpose of increasing the amount of flow of 
the sap, and, therefore, the quantity of resin ; after collection it is 
taken to Mogador for export to Europe. 

Sandarac occurs in the form of short cylindrical pieces or tears, 
which are sometimes agglomerated together; in colour it is a 
yellowish white ; it is a comparatively hard resin, being about 
equal to kauri or rosin in this respect ; it melts easily at about 
300 F. (150 C.), and breaks with a clean, lustrous fracture. 
The specific gravity is 1*038 to 1*044. 

It is soluble in alcohol and ether, partially soluble in benzol, 
petroleum spirit, and turpentine, but very slightly soluble in oil 
before fusing, yet readily so after fusing. It is used in making 
pale spirit varnishes, and is valued on account of the hardness 
and lustre of the coat it forms. 

Rosin or Colophony. The name of this body is spelled rosin 
or resin ; the former is more in accordance with the pronunciation 
of it and will be adopted here; the latter is liable to confusion 
with the generic name for the group of resins. Colophony is 
rarely used. Rosin is obtained in the distillation of turpentine 



ASPHALT UM. 403 

from gum thus (see p. 361) and is left behind in the still after the 
turpentine has come over; the melted rosin is run from the stills 
into moulds or barrels to cool. Rosin comes into the market in 
the form of large pieces, generally homogeneous, varying in 
colour and transparency; the best quality is known as " window 
glass " rosin, and is a pale amber in colour, perfectly clear and 
transparent; "common" rosin is much darker in colour but is 
still homogeneous, clear and transparent; "black" rosin is very 
dark, almost approaching a black, and opaque. These varieties 
are dependent upon the quality of the original resin from which 
the rosin has been made ; the so-called " virgin " resin gives the 
best rosin, while the resin collected later on in the season gives 
common rosin, and the scrapings of the resin from the bark, <fec., 
give black rosin. Rosin is slightly heavier than water, its 
specific gravity being 1-044 to I'lOO ; it easily melts; at about 
177 F. it softens, and at 212 F. it is quite fluid; "window- 
glass" rosin forms a clear, limpid, yellow liquid. It is insoluble 
in water ; is soluble in about eight times its weight of alcohol ; 
in benzol, coal-tar naphtha and acetone in almost any pro- 
portion; soluble in turpentine, ether, and most oils. It is also 
soluble in boiling solutions of the alkaline carbonates or hy- 
droxides, becoming hydrolised and taking up the elements of 
water to form abietic acid, with which the alkalies combine to 
form the rosin soaps so largely employed in the soap industry. 
Rosin is a mixture of two acids bodies, pinic and sylvic acids, 
which are isomeric and have the formula C 20 H 30 O 2 . Some 
authorities consider that the composition of rosin is not that of 
a true acid, but an anhydride, abietic anhydride, which will take 
up water to form abietic acid. 

When distilled rosin yields a small quantity of acid water, 
spirit (see p. 372), a heavy oil (see p. 357), and a residue of pitch. 
It is used in making common oil varnishes, in making some cheap 
spirit varnishes, and in naphtha varnishes. By itself it leaves 
rather a hard, brittle, and lustrous coat, but tempered with some 
oil or soft resin it makes a durable varnish, not, of course, ap- 
proaching the copals or kauri in this quality. 

A solution in hot alcohol deposits crystals of sylvic acid on 
cooling. Nitric acid converts rosin into terebic acid, H C 7 H 9 O 4 , 
which is soluble in water. 

Asphaltum. Asphaltum or native bitumen is used in the 
varnish manufacture in the production of black varnishes, blacks, 
japans, <fec. It was originally obtained from the shores of the 
Dead Sea, and this variety is specially designated Bitumen of 
Judea and Egyptian asphaltum, but it is imported from other 



404 VARNISHES. 

places, Altona in Albania, Coxitambo in South America, and 
Barbados, while there is an almost inexhaustible supply in the 
great lake of Trinidad. Asphaltum, when pure, is a blackish- 
brown solid, breaking with a conchoidal fracture, the surfaces 
being bright and lustrous. At 100 C. (2 1 2 F. ) it melts to a black 
liquid having a strong pitchy odour. It burns with a bright 
somewhat smoky flame. In water and alcohol it is insoluble. 
It dissolves readily in coal-tar naphtha, but not so readily in 
turpentine. Its specific gravity varies from 1 -00 to 1 -20. 

When used alone, simply dissolved in naphtha or turpentine, 
it dries with a gloss, with, however, a very brittle coat that in time 
begins to crack very much ; on this account asphaltum, formerly 
much used by artists, has of late years been abandoned for oil 
painting. 

In making japans and black varnishes its brittleness has to be 
overcome by the use of oil and other gums. Asphaltum is 
generally considered to be a product of the decomposition of 
animal and vegetable organic matter. 

Artificial Asphaltums are now made by mixing together in 
various proportions coal-tar pitch, wool pitch, rosin pitch, and 
other pitches obtained in the distillation of animal and vegetable 
oily products. These are very useful in making cheap black 
varnishes, but considerable care must be taken in their selection, 
as many of the pitches, especially those from petroleum and 
paraffin, will not dry. 

ETHEREAL-VARNISH RESINS. This is an important 
group p resins, which are more or less soluble in such solvents 
as benzol, coal-tar naphtha, turpentine, ether, acetone, &c., and 
which are used with these solvents to make varnishes for special 
purposes. This group includes : 

All oil varnish resins after they have been fused. 
Dammars. Rosin. 

Mastic. Manila copal. 

Sandarac. 
x - 

/Some of these have already been described. 

V- Dammar . Under the name of Dammar there is imported into 
this country, for making varnishes, several kinds of resins, prin- 
cipally from Siam, and which do not differ very much from one 
another in their properties. 

1. Singapore dammar , also known as white dammar, is the 
true dammar. It is the produce of the Amboyna pine, Dammara 
orientalis, a large tree indigenous to Malacca, Java, Sumatra, 
Borneo, and the Malaccas, growing in the hill districts ; in Java 



ETHEREAL-VARNISH RESINS. 405 

it is cultivated to a small extent. The supply principally comes 
from Java through Singapore. The resin exudes from certain 
excrescences which grow a short distance above the root of the 
tree ; in Sumatra and other places the resin exudes out in large 
quantities, while in other localities the natives make incisions in 
the excrescences to promote the flow of the resin; large quan- 
tities of the resin are also found in the river courses, having 
probably fallen from trees growing on the sides of the rivers. 
The resin exudes also from the upper portions of the trees, 
branches, tfcc. ; the resin from the roots is usually in the form 
of large rounded knotty pieces, while that from the branches is 
more or less stalactitic in form. 

Singapore dammar comes into commerce in the form of nodules, 
varying in size from J an inch to 1J inches, sometimes larger, 
covered with a powdery crust ; the interior is usually clear and 
transparent and is nearly white in colour, at the most it has a 
faint straw tint. It is friable and breaks easily with a powdery 
fracture. It is not quite so hard as copal, but is harder than 
rosin ; when fresh it has a faint agreeable balsamic odour, which 
disappears on keeping. Its specific gravity is 1*062 to 1'123. 
It is soluble in turpentine, ether, petroleum spirit, chloroform, 
and in oil. That from Batavia dissolves more freely than the 
Singapore variety. The melting point of both is about the same, 
260 to 300 F. It is used in making varnishes for coach and 
cabinet makers, for paper, for pictures, and it is used dissolved in 
benzol for mounting microscopic objects. Dammar gives a pale, 
hard varnish with a fair amount of lustre. It has the defect of 
being rather friable, so that if, when dry, the coat is rubbed with 
the fingers it becomes powdery. 

2. Rock Dammar. This resin, which is almost indistinguish- 
able from the last variety, is the produce of two species of Hopea 
viz., Hopea odorata, which grows in Burmah, about Rangoon, 
Pegu, Martaban, and Tenasserim ; and Hopea micrantha, a native 
of the Malay States of Malacca, Sumatra, Borneo, and Labuan. 
There are great differences between the resins yielded by these 
two trees ; that from odorata generally occurs in rounded pieces 
about the size of walnuts, and is pale in colour ; colourless pieces 
are often found. The micrantha resin is rather darker and a little 
harder. Rock dammar is soluble in turpentine, and other solvents, 
and is of equal value with the Singapore and Batavian dammars. 

3. Sal Dammar. This is the produce of the Sal tree, SJwrea 
robusta, which grows on the southern flanks of the Himalaya 
mountains along nearly their whole extent ; it is also found 
among the hills of West Bengal, in Borneo, Su 



UNIVERSITY 



406 VARNISHES. 

Malaccas ; so that its range is a wide one. It occurs in long- 
pieces of a stalactitic character, rather brittle, mostly of a pale 
cream colour, more or less opaque and striated as if the pieces 
were formed by the separate flow of different streams of liquid 
resin, each of which solidified before the succeeding one had 
begun to run. Its specific gravity ranges from about 1-097 to 
1-123. It is soluble in ether, benzol, and turpentine, partially 
soluble in alcohol ; the solutions are not quite clear, but always 
have a more or less turbid appearance. 

It is not often met with in the English market. It is used in 
making paper varnishes and tracing paper, as, when dissolved in 
turpentine, it gives a good, pale, hard varnish which dries well 
and is elastic. 

4. Black Dammar. Black dammar, known in India as kala 
dammar, is the produce of several species of Canarium trees, 
principally Canarium strictum which grows in the Tinevelly 
district. The resin is collected in a very different manner from 
that adopted with other resins ; the natives in the hot season 
make a number of vertical cuts just above the base of the trunk 
and then set fire to the tree below these cuts ; by this means they 
kill the tree, which is now left for two years, at the end of which 
time a quantity of resin will have exuded from the trunk ; this 
resin is collected in the months of February and March. The 
resin flows for some years after the tree has been killed. In the 
Coimbatore district a somewhat different system is used to collect 
the resin ; firewood is piled round the tree to the height of about 
1 yard ; this is fired and allowed to burn out ; the resin sub- 
sequently exudes from the tree to about as high as the flames of 
the fire extended. The flow lasts for 10 to 12 years, and principally 
occurs during the months of November and April ; generally, the 
resin is collected in January. 

A tree will yield 200 Ibs. of resin. Black dammar occurs in 
large black or brownish-black pieces, which are opaque when 
viewed in the mass, but in thin slices are transparent; it is 
homogeneous and vitreous in structure, breaking with a clear, 
conchoidal fracture. Specific gravity, 1-090. It is not very 
soluble in cold alcohol, but dissolves in hot alcohol ; it is soluble 
in turpentine. On distillation it yields a large quantity of oil 
resembling rosin oil. 

It is used in India for making varnishes, but in this country 
it is rarely seen, and it is doubtful whether there would be much 
demand for it, as, on account of its colour, it cannot be used for 
many kinds of varnishes and for some purposes it cannot compete 
with the cheaper rosin. . 



ETHEREAL-VARNISH RESINS. 407 

Mastic. This resin is the product of the lentisc tree, 
Pistachio, lentiscus, which grows in all countries washed by the 
Mediterranean ; Spain, Portugal, Italy, Greece, the islands on the 
Greek, Turkish, and Levant coasts, North Africa, &c. The prin- 
cipal portion of the mastic comes from the district around Chios, 
one of the principal islands of the Greek archipelago. The 
mastic tree is shrubby, growing to a height of from 4 to 5 feet ; 
the bark contains numerous resin vessels from which the resin is 
exuded in fairly large quantities on making excisions. About the 
months June to August the natives of the mastic district make 
many vertical incisions in the bark of the stem and branches, 
keeping these open during the period mentioned ; the resin flows 
freely and soon becomes dry and hard ; about two or three weeks 
after the cutting the resin is collected in small paper baskets, great 
care being taken to keep the resin clean ; with the same object 
the ground below the tree is kept free from loose dirt, so that any 
resin which may accidentally fall from the tree may not get 
dirty. The lower branches of the tree frequently exude resin 
spontaneously ; this is considered to be of a -superior quality and 
is kept apart from the rest. A tree in good condition will yield 
from 8 to 10 Ibs. per annum, but in the rainy season only f lb., 
showing a considerable difference in the yield, which is much 
less in such seasons. 

Mastic is sent into commerce in several forms : 1st, Cake, 
which is in the form of large pieces, and is the best quality of 
mastic ; this variety is largely used as a chewing gum in Turkey. 
2nd, Large mastic ; this variety is mostly used as a chewing gum. 
3rd, Small mastic ; this occurs in small tears, and is the variety 
mostly met with in this country and which is used for making 
varnishes and for other industrial uses. 

Mastic comes in the form of small tears of from J to ^ an inch 
long, mostly of a pale yellow colour, but sometimes they have a 
faint greenish tint ; old specimens are darker than fresh samples. 
The outer surface of the tears is often powdery and the tears 
appear to be opaque ; sometimes the tears are quite clear and 
transparent. Mastic is rather brittle and breaks with a conchoidal 
glassy fracture. Fresh mastic has a pleasant balsamic odour. It 
softens readily when placed in the mouth, a character distinguish- 
ing it from sandarac, which resembles it in form. Its specific 
gravity is about 1-056 to 1-060; it melts at from 105 to 120C. 
(221 to 248 F.) ; but softens below the temperature of boiling 
water. It is soluble in turpentine, alcohol, chloroform, and 
acetone, but not in petroleum spirit. It is used in making 
varnishes for pictures ; a combination of mastic varnish and 



408 VARNISHES. 

linseed oil forms the peculiar artists' medium known as 
megilp. 

Manila Copal. This resin is the product of various species 
of trees growing in the Philipine Islands. It is gathered by the 
inhabitants and exported, chiefly from Manila, the principal town 
of the islands ; hence it is called Manila copal. The use of the 
term copal is misleading, as it does not resemble the copals in 
its properties. The only similarity is that, like these, it is of 
vegetable origin. 

Manila copal occurs in small pebble -like pieces of a pale 
brownish-yellow colour, varying slightly in different samples. 
Its specific gravity is about 1O62. It differs very much from 
the true copals in all its properties ; it is much softer ; melts at 
from 230 to 250 F. ; and at a temperature a little above this 
begins to distil over, the distillate consisting of both acid water 
and an oil having a very empyreumatic odour. 

Manila copal swells when placed in petroleum spirit, and a 
little dissolves ; it is soluble in ether, and partially soluble in 
benzene, chloroform, and turpentine ; in alcohol it dissolves to a 
turbid solution. Manila copal which has been melted is much 
more soluble than fresh copal, the fused gum being freely soluble 
in all solvents except alcohol, which does not dissolve it any more 
freely than the fresh resin. It is used in making spirit varnishes, 
more for the purpose of giving elasticity to other resins than for 
any brilliancy and hardness it possesses of itself. It leaves a faint 
durable coat behind it, and is much used in the preparation of 
enamel paints. 

SPIRIT-VARNISH RESINS. This group of resins com- 
prises those which^are soluble in alcohol or methylated spirit and 
they form an important and valuable group. The spirit-varnish 
resins include : 

Lac. Mastic. 

Manila copal. Rosin. 

Sandarac. Melted resins. 

Some of these have already been described, leaving but a few 
that require to be dealt with. 

Y Lac is a most important product from many points of view. 
In its origin it differs from the resins previously described. Lac 
is a resinous incrustation found on the twigs of many species of 
Indian trees, the number and variety of which has not as yet 
been properly ascertained. The following is a list of the most 
important lac trees : Palas or dhak tree, Butea frondosa; the 
peepul tree, Ficus religiosa; the koosum tree, Schleichera trijuga; 



SPIRIT-VARNISH RESINS. 409 

Acacia arabica, Acacia catechu, various species of Croton (lacci- 
ferum, draco, sanguiferum), Butea superba, various species of 
Ficus (elastic a, cordifolia, venosa, villosa, indica, glomerata, &c.), 
Mimosa cinerea, <fcc. All the lac trees have a very gummy or 
resinous sap. The lac is not a direct product of the tree, but is 
formed from the sap by the female of the lac insect, Coccus lacca. 
The insect punctures the bark of the tree and commences to 
secrete the lac, forming it into cells in which it lays its eggs ; the 
insect becomes in time completely imbedded in the lac, breathing 
by means of fine filaments which it sends to the surface for that 
purpose ; when it has laid its eggs it dies ; after the young insects 
are hatched they puncture the lac and swarm over the twig or 
tree branch ; the males impregnate the females, which latter then 
proceed to secrete lac as their ancestors did before them ; the 
insects do not move from the portion of the tree on which they 
first swarmed. The tree supplies nourishment to large quantities 
of lac insects, but at the expense of its own vitality ; for, after a 
time, it begins to decay and then it ceases to be able to support 
any more insects. The distribution of the insects from place to 
place is probably, in the absence of any effort by the insects 
themselves, effected by the agency of other insects and of birds 
who carry the young from tree to tree ; of late years artificial 
propagation has come into existence. 

The principal portion of the lac of commerce is grown in India, 
but it is also obtained from other Asiatic countries. The lac of 
Siam has a great reputation for quality. It is also found in 
Ceylon, Burmah, China, Malay archipelago, and other localities. 
In India lac grows principally in the province of Bengal, the 
capital of which is a great emporium for lac in all its states ; in 
this province the jungle districts of Chota Nagpore, Orissa, and 
Beerbhoom, are the chief localities. It is collected twice a year, 
from about the middle of October to January, and from the middle 
of May to the middle of July. In the Scinde State it is found 
abundantly in the forests surrounding Hyderabhad, where it chiefly 
grows on the babool tree, Acacia Arabica ; the tree, however, is 
not attacked by the insects while it is in the full vigour of its 
growth, but as soon as it begins to wither the insects attack it 
and thrive well upon it, eventually killing it. The period of 
gathering the lac extends from October to the following April. 
It is mostly consumed locally, being used to produce the famous 
lacquered ware for which Hyderabhad is noted. Assam is a great 
lac country, and lac is collected in large quantities ; much is used 
locally, but still a large quantity finds its way to Calcutta for 
. export. In the Central Provinces large quantities of lac are pro- 



410 VARNISHES. 

duced, in the east districts especially. Jubbulpore is a great 
centre for lac ; the district itself supplies a great deal, and much 
comes into the city from Rajpore, Bilaspore, Mundla, and Sangor; 
most of it is used locally in the manufacture of ornaments of 
various kinds, and the rest is sent to Bombay for export. Sum- 
bulpore and Mirzapore are also places of note in the lac trade. 
Sobhapore, in the Hoshungabad district of the Central Provinces, 
is a large centre of the lac trade, the lac coming into it from 
Futtehpore, Bankheri, the Nerbuddha Hills, Nursinghpore, and 
other places. The towns of Hoshungabad and Babai are also 
places of trade in lac. From Oudh comes supplies of lac which 
is found principally in the forests of the south-eastern districts, 
where it grows chiefly on the Ficus religiosa. In the Punjab lac 
is found in large quantities, although of inferior quality ; it is 
sent to Calcutta for export. There are many other localities 
where lac is obtained in large or small quantities ; some of it is 
used locally for a great variety of purposes, and the rest is sent 
to one of the principal seaports for export to Europe. 
Lac comes into commerce as 

1. Stick lac. 4. Button lac. 

2. Seed lac. 5. Garnet lac. 

3. Shellac. 

Another lac product is lac dye, which at one time was largely 
imported into Europe for dyeing purposes, but since the intro- 
duction of the coal-tar colours it has completely fallen into disuse. 

1. Stick lac is the crude product just as it is taken from the 
trees ; it rarely comes into English commerce, as it does not pay 
to export useless twigs of trees. Stick lac is in the form of short 
pieces (about 2 to 3 inches long) of the twigs with the lac in- 
crusting them ; it is cut into these lengths for convenience of 
carrying, <fec. Stick lac is sent from the forests and jungles, 
where it is collected, to the different towns for manufacture into 
lac products. The process of manufacture is very simple, and is 
carried out in a very crude manner, although of recent years im- 
provements have been made with the view of increasing both the 
quantity and quality of the products. 

The process of lac manufacture consists in first separating the 
lac from the woody portion of the stick lac ; this is done by 
placing the stick lac on a table and passing a roller over it ; the 
lac being brittle is broken and separated from the wood almost 
entirely ; the little that is left adhering to the wood is picked off 
by hand. The wood is thrown away, the lac is taken, broken 
into small pieces about J inch in size, placed in large tubs with 



SPIRIT-VARNISH RESINS. 411 

warm water, and worked by the workmen getting into the tubs 
and treading ; this process extracts the colouring matter from the 
lac, and is continued until the wash waters remain clear. The 
lac left behind is known as "seed lac." The liquor is boiled 
down dry, and the resulting solid mass is formed into cakes and 
sold as " lac dye." 

2. Seed lac. This is the partly manufactured product obtained, 
as described above, by treating stick lac with water ; this is some- 
times sold for various purposes, but it is mostly manufactured 
into the other lac products. 

3. Shellac. This is the principal lac product and the variety 
which is mostly used in this country for varnish making. Shellac 
is made from seed lac as follows : The seed lac is dried and 
placed in large bags made of cotton cloth of a medium texture. 
Two men take hold of the bag of seed lac, one at each end, and 
hold it in front of a charcoal fire ; the heat of the fire soon melts 
the lac, which flows out of the bag, the men assisting the flow by 
twisting the bag from each end in opposite directions, so as to 
squeeze the liquid lac out of the bag ; the molten lac drops into 
a trough placed in front of the fire. A cylinder (made of different 
materials at different localities) of wood, with the upper half 
covered with brass, or it may be made of porcelain or of the 
finely polished stem of the plantain. This cylinder is set in a 
somewhat inclined position, and the operator, taking up a ladleful 
of the molten shellac from the trough, pours it on the upper sur- 
face of the cylinder, while an assistant, by means of a leaf of 
plantain, spreads the melted material over the surface of the 
cylinder. It soon sets, when, by means of a knife, it is stripped 
from the cylinder, and is then ready for sale as " shellac." 

The best quality of shellac has a pale and bright orange colour, 
and is hence known as "orange shellac;" but in the market 
many qualities are recognised distinguishable from one another 
by their colour, by their freedom from dirt and grit, and by their 
transparency. 

4. Button lac is only different from shellac in form. Instead 
of being made into thin sheets the melted shellac is poured on to 
plates in such a manner that it sets in the form of large round 
flat pieces, which, owing to their thickness, appear of a dark 
brown colour; but are of a dark ruby colour on being looked 
through. 

5. Garnet lac is very similar to button lac, but is made into 
thick flat pieces, which, in colour, resemble those of button lac. 
Usually the quality of lac made into button or garnet lac is not 
so good as that from which orange shellac is made. 



412 VARNISHES. 

For making varnishes either shell, button, or garnet lac may be 
used, but the latter two are only used for making the commoner 
kinds of shellac varnish, where colour is not so much an object; 
for the best qualities of spirit varnishes only orange shellac is 
used. 

Of late years some improvements have been adopted in one or 
two large lac factories in India. The lac is better washed, 
whereby more colouring matter is extracted from the crude lac; 
then, the appliances for melting and straining and converting the 
lac into its shell form are considerable improvements on the 
primitive methods described above. 

Lac comes into commerce in three forms shellac, in thin flakes 
of an orange colour, varying a little in shade and transparency; 
button lac, in large round flat masses of a dark colour; and as 
garnet lac, in irregularly shaped flat pieces of a dark ruby colour. 
It is rather brittle and easily broken up into small pieces. 
Occasionally it is artificially coloured with orpiment or mixed 
with rosin, but such adulterations are rare. 

Lac is incompletely or only partially soluble in alcohol or 
methylated spirit, forming a turbid brownish-orange solution, 
which is largely sold as French polish and varnish for cabinet 
,and other work. 

Lac is only partially soluble in ether, chloroform, and turpen- 
tine, while it is insoluble in petroleum spirit. It is soluble in 
solutions of caustic potash, and of caustic soda to dark red solu- 
tions. In borax solution and in weak ammonia it is also soluble, 
and such solutions are sometimes used as water varnishes. One 
point of interest in the solubility of shellac in such alkaline 
liquors is that the colouring matter is first dissolved away from 
the resin proper, leaving the latter of a pale colour ; this property 
is taken advantage of in preparing white shellac. Chlorine 
passed through alkaline solutions of lac throws down the resin 
free from colour. Lac has a specific gravity of I'll 3 to 1'214, the 
darker varieties being the heavier. 

Crude stick lac freed from woody matter contains 66 -67 per 
cent, of resin, 6 per cent, of wax, 6 per cent, of gluten, and 10 '8 
per cent, of colouring matter. In shellac five distinct resins have 
been separated (1) resin soluble in alcohol and ether; (2) resin 
.soluble in alcohol, but insoluble in ether; (3) resin slightly 
soluble in alcohol ; (4) a crystallisable resin ; and (5) an uncrys- 
tallisable resin. These constitute about 90 per cent, of the 
shellac. There are in addition (6) fatty matter; (7) wax; (8) 
gum ; and (9) colouring matter. 

Bleached or White Shellac. Shellac may be bleached in two 



SPIRIT-VARNISH RESINS. 413- 

or three ways. One method is to boil ordinary shellac in a weak 
solution of carbonate of potash, and when dissolved passing a 
current of chlorine through it ; the lac precipitated is collected, 
melted under water, and then, while soft, pulled so as to give it 
a fibrous satiny appearance. Another method is to boil the 
shellac in a weak solution of potash, and, while melted, pulling 
and working together until the desired whiteness has been 
attained. Then the shellac is remelted and repulled in clean 
warm water. White shellac is sold in the form of long cylindrical 
pieces having a fibrous satiny appearance. It is used for making 
white varnishes and for other purposes where a white shellac is 
required; as it is sold in rather a wet condition it is necessary to 
dry it before using it. It is sometimes considered that white 
shellac deteriorates on keeping, but this is somewhat doubtful, 
and there is no apparent reason why it should. 

Elemi. Under this name several varieties of a resinous matter 
come into commerce which are used as a softener or toughener 
for varnishes. 

1. Manila Elemi. This comes from the Philipine Islands and 
is the product of a tree known as Canarium commune to botanists, 
which principally grows in the island of Luzon ; the supply is 
sent through Manila. This variety of elemi is white when quite 
pure and of good quality, although some samples have a grey 
appearance and others appear to be composed of two or three 
sorts of resins. The resin is soft and has a granular appearance ; 
when exposed to the air for some time it becomes hard, owing to 
the evaporation of the volatile oil present in freshly-gathered 
elemi. The odour is slightly turpentiney. When distilled this 
elemi yields about 10 per cent, of an oil resembling turpentine 
in its composition. Elemi begins to soften at about 75 to 80 C., 
and is quite liquid at 120 C. It is soluble in alcohol and in 
most other solvents. It is used in varnishes to give elasticity or 
toughness. 

2. Mexican Elemi. This variety is obtained from a tree known 
as Amyris elemifera. In its essential properties it resembles 
Manila elemi, but is rather darker coloured and harder in appear- 
ance and consistency. It is not very common. 

3. Brazilian elemi is supposed to come from trees belonging to 
the genus Idea, but is not an article of regular importation into 
this country. 

4. Mauritius elemi is supposed to come from the tree Colophonia 
Mauritiana, and is stated to resemble Manila elemi in appearance 
and properties. 

Benzoin. Gum benzoin or gum benjamin is a balsamic resin 



414 VARNISHES. 

which exudes from Styrax benzoin, a native of Sumatra, Java, 
Borneo, Siam, Laos, and other places in the same region. The 
commercial product comes principally from Siam and Sumatra. 
The Malays know it by the name "kaminian." In the coast 
regions of North and East Sumatra it is cultivated from seed. 
In about seven years it obtains a diameter of 6 to 8 inches and is 
then ready to be cut. The natives make incisions in the tree and 
the resin exudes from them very freely, each tree yielding about 
3 Ibs. annually ; for the first three years the product is of first- 
class quality, of a yellowish- white colour and soft with a fragrant 
odour; after the third year the product is darker in colour, harder 
and not so fragrant ; after about nine years it is not worth collect- 
ing. In the interior regions the resin is collected from wild trees. 

The benzoin which comes into England is imported almost 
entirely from Siam and Sumatra. There are some small differences 
in the appearance and properties of these two varieties of 
benzoin. 

Siam benzoin comes into commerce in the form of agglutinated, 
flattened, somewhat opaque, milk white tears or in large agglom- 
erations consisting of white masses distributed through an amber- 
coloured, rather translucent matrix. It is brittle, and has a 
strong vanilla-like odour, which is very characteristic. It is 
readily softened by heat ; it is quite soft at 75 C. and fluid at 
100 C. 

Sumatra benzoin is rather greyer, and is in the form of an 
agglomerate mass with white tears distributed through a darker 
translucent matrix. Its odour is not so strong as that of Siam 
benzoin and it does not melt so easily ; it contains more benzoic 
acid, which comes off in vapour when the resin is heated. In all 
other properties the two resins are identical. 

Benzoin has a peculiar fragrant odour, and is slightly heavier 
than water; its specific gravity being about 1'092 to 1-145, 
although some authorities give rather higher figures than these. 
It melts at a gentle heat and gives off white vapours of benzoic 
acid with a small quantity of volatile oil. Alcohol dissolves most 
of it ; ether has a slightly less solvent action ; while turpentine 
and petroleum spirit dissolve very little of it. Treated with 
sulphuric acid, benzoin or an alcoholic solution turn bright red, 
while with ferric chloride a green colour is produced. 

Benzoin finds a small use in varnishes, chiefly on account of 
the odour which it imparts to the varnish ; it is also used in 
making perfumes, incense, &c. 

3rd. G-UMS. The true gums, such as gum arabic (the type of a 
true gum), gum tragacanth, and a few others are, like the resins, 



GUMS. 415 

exudations from trees, and are collected much in the same manner. 
They differ from resins in one or two important particulars. In 
the first place, they are more or less soluble in water ; some, like 
arabic or acacia gums, are completely soluble ; others, like gum 
ghatti, are partially soluble; while others again, like tragacanth, 
are not properly dissolved by water, although acted upon by that 
vehicle. The true resins are quite insoluble in water. On the 
other hand, the gums are quite insoluble in alcohol and other 
similar solvents. 

The gums, which belong to the third group of varnish mate- 
rials, are almost exclusively used for making water varnishes, or 
(as in water-colour painting and in some kinds of distemper work) 
as fixing agents to fasten the pigment on to the work. 

Besides the gums proper there are used in making water var- 
nishes gelatine or glue, dextrine or British gum and albumen. 

Gum Arabic. The name of this gum is a misnomer, because 
it would indicate that it comes from Arabia, whereas but little, 
if any, now comes from that country ; probably in early times it 
may have been obtained from that country, or, at least, imported 
through it and hence the term "arabic" arose. It is also known as 
gum acacia, from the trees which yield it; but gum arabic, despite 
its error, is the name by which it is best known in the trade, and 
therefore it will be retained here. 

The species of A cacia are profusely distributed throughout the 
tropical parts of Africa, Asia, and Australia, and are all gum- 
producers. The gums vary a little in appearance and properties, 
but all are more or less soluble in water, and give with that 
solvent a strongly adhesive mucilage useful for a great variety of 
purposes. These gums are distinguished in the trade by terms 
descriptive either of their place of origin or of their quality, such 
as picked Turkey, white Sennaar, Senegal, Cape, Mogador, Indian, 
Ghatti, Wattle, brown Barbary, <fcc. it is not necessary to enter 
into a very full description of all the varieties of gum arabic, as 
these are of little importance in connection with the topics dealt 
with in this book. 

Picked Turkey or white Sennaar arabic is the produce of A cacia 
Senegal, a tree growing in the Upper Nile regions and in Kordofan, 
where it is collected by the natives and shipped to Egyptian ports 
for exportation. Gum Senegal is the produce of the same species 
of Acacia^ and is collected in the French province of Senegal. 
This variety of gum is very varied in quality, ranging from a fine 
white gum to a dark somewhat reddish gum. It is exported 
from Senegal almost entirely to Bordeaux, and but little comes 
into England. The best qualities are used for pharmaceutical, 



416 VARNISHES. 

confectionery, and other purposes, while the common qualities 
find a use in textile industries, varnish-making, and for making 
mucilages. Suakim or Soudan gum is a variety of gum arabic 
derived from two other species of Acacia, A. stenocarpa, the talch 
or talha tree of the Arabs, and A. Seyal, the soffar tree of the 
natives. This variety is collected in the tipper Nile regions and 
in the districts through which some of its tributaries flow. It is 
sent into commerce either through Khartoum, or, more largely, 
through Suakim, a port on the Red Sea. Yery large quantities of 
this gum come into the English market, and it forms the main 
source of the gum arabic used for general commercial purposes. 
Its quality varies somewhat, from a good fine white gum to a dark 
discoloured sort. Morocco gum arabic is said to come from Acacia 
gummifera, but its source is somewhat uncertain. It is only 
collected in small quantities, and is chiefly exported through 
Mogador. It occurs in large globular tears of a brownish tint, 
from which circumstance it is sometimes known as brown Barbary 
gum. Cape gum. The doornboom, Acacia horrida, one of the 
commonest trees of South Africa, yields a large quantity of a 
brownish gum, which differs from gum arabic in not being so 
completely soluble in water. It is used in the Cape Colony and 
surrounding districts in place of gum arabic, and is exported to a 
small extent. East Indian gum. Much of what is sold as East 
Indian gum arabic is African produce, exported by way of Aden 
and Bombay, and does not difier, therefore, from Suakim gum in 
quality. Several species of Acacia grow in India and yield gum, 
which is collected and used locally, but a little finds its way 
into this country. Not much is known about these Indian gums. 
The Acacia arabica grows in Bengal, the Deccan, and Coromandel. 
The Acacia catechu, the cutch tree, yields a gum of rather dark 
colour, but otherwise equal to gum arabic in quality. Acacia 
speciosa yields a gum, known in India as the siris gum, which is 
of good quality. From other Indian Acacias gums are obtained 
in smaller quantities. Besides the Acacias other trees yield gums 
which sometimes find their way into the English market, such, 
for example, as gum ghatti ; these gums are not so good as true 
gum arabic. The gums from the Australian wattles (which are 
various species of A cacia, the principal tree being the green wattle) 
A. decurrens, A. pycnantha, A. homalophylla, A. harpophylla, and 
A. Bidwilli, are of smaller importance. The quantity of gum 
obtained is rather larger than from African trees, and is of good 
quality. 

The following description is nearly applicable to all the above 
varieties of gums. Gum arabic occurs in roundish or ovoid or 



GUMS. 417 

even vermicular masses of various sizes ; the surface always has 
a glistening appearance, the colour varies from an almost colour- 
less gum to a faint reddish or brownish tinted gum. While some 
pieces are transparent, many are simply translucent, and some 
are opaque; they are brittle and easily friable, although this 
property is a variable one, the Australian gums being less friable 
than the others; gum Senegal is generally the most friable. 
These gums are quite free from odour, and their taste is slight. 
They are quite soluble in water; the best qualities take about 1J 
times their weight of water to form a thick viscid, highly adhesive 
mucilage, which is insoluble in alcohol; hence the addition of 
alcohol to the aqueous solution causes the precipitation of the 
gum. A solution of subacetate of lead added to the aqueous 
solution results in the formation of an opaque white jelly. Iodine 
does not produce any colour in solutions of gum arabic. Nitric 
acid converts them into mucic and oxalic acids; while boiling 
with sulphuric acid converts them into dextrine and sugar. 

Gum arabic consists essentially of arabine, a compound of 
arabic acid (C 12 H 22 O n ) with lime; besides this compound gum 
contains traces of sugar, dextrine, colouring matter, tannin, and 
mineral matter. 

Gum Tragacanth. This gum is an exudation from various 
species of Astragalus, of which the most common is A. gum- 
mifera, growing in Lebanon, Syria, Central Asia Minor, and 
Armenia. Other species of Astragalus growing in Asia Minor, 
Judea, Syria, and Persia also yield gum tragacanth, which comes 
into commerce principally through Baghdad. Gum tragacanth 
occurs in two forms (1) leaf gum, in strips of about to J inch 
wide, and 2 to 3 inches long; (2) "vermicelli" gum, in long round 
pieces. It is of a dull greyish colour, without odour or taste. 
Placed in cold water it does not dissolve, but swells up into a 
gelatinous mass, which, when boiled for some considerable time, 
gradually passes into a kind of solution forming a very jelly- 
like fluid. It is readily soluble in alkaline liquids, but not in 
alcohol. It consists essentially of a compound known as basso- 
rine (C 6 H 10 O 5 ), which is insoluble in water and alcohol. Be- 
sides this there is generally a small quantity of ordinary arabine 
soluble gum. Gum tragacanth, owing to its dull appearance, is 
not used in varnish making, but finds a use as a thickener in 
calico-printing, and for a few other purposes. 

Besides these two gums, other kinds of gums are occasionally 
met with in small quantities in the London market. Some 
resemble gum arabic in their properties, and can be used for the 
same purposes; others resemble gum tragacanth; while some, 

27 



418 VARNISHES. 

such as the kuteera gum of India, appear to contain both soluble 
and insoluble gums. 

Dextrine or British Gum. This product sometimes occurs 
in water varnishes. It is prepared by acting on starch either 
by heat alone or by heating with a small quantity of acid; 
the former method is most commonly used in England, and gives 
the best dextrine; the acid method is the one by which German 
dextrine is mostly prepared. Dextrine comes into commerce as 
a powder varying in colour from a pale yellow, "white," to a dark 
brownish-yellow, "yellow" dextrine; intermediate varieties being 
known as "canary." Pure dextrine is quite white, tasteless, and 
free from odour, the commercial varieties have a sweetish taste 
and slight odour. Pure dextrine is quite soluble in water; the 
solution reduces Fehling's solution and gives a brownish colour 
with iodine. The solubility of the commercial dextrines varies 
considerably; some are quite soluble, others are only partially 
soluble, as they still contain some unchanged starch. These solu- 
tions reduce Fehling's solution, and often give a blue colour with 
iodine. The solutions have more or less adhesive properties; 
hence, dextrine is largely used for adhesive purposes as a substi- 
tute for gum arabic. Dextrine dries with a fair amount of gloss 
or lustre; hence it is used in making water varnishes. There 
is a considerable difference in the commercial varieties of dextrine 
in regard to both their adhesive and lustreing properties, some, 
while giving a thick mucilage, have little adhesive property, and 
others dry with little gloss. On boiling with water, dextrine 
gradually loses its adhesive properties, which change is hastened 
by the addition of small quantities of acid; this is due to the 
transformation of the dextrine into sugar. 

Gelatine. This is an animal product obtained by boiling the 
cartilaginous tissues of animals, bones, &c., in water, and evapor- 
ating the solution down and drying the product. The character 
of the product varies very considerably according to the material 
from which it is extracted and the care with which the process of 
manufacture is carried out. The finer qualities known as gela- 
tine are generally in the form of thin sheets of 7 or 8 inches in 
length by 2 or 3 inches in breadth ; quite white, or rather colour- 
less, and transparent sheets are flexible and tough, not being 
readily broken. The commoner qualities are known as glue, and 
vary very much in appearance; the better qualities are made in 
thin sheets of the same size as the gelatine sheets above men- 
tioned, but a little thicker; they are of a pale yellowish- white 
colour; the inferior qualities are generally made in sheets of 
about 6 inches square of varying thickness and usually of a 



SOLVENTS. 

reddish-brown colour, more or less transparent. Glue is valued 
on account of its adhesive properties and is used in many arts. 
Gelatine is used in making varnishes and for fine work, where a 
colourless adhesive is required. Gelatine when placed in cold 
water swells up into a jelly-like mass; when boiled with water it 
dissolves, forming a thick viscid solution having very strong 
adhesive properties; on cooling, this solution forms a jelly-like 
mass. It takes but a small proportion (from \ to 1 per cent.) of 
gelatine to cause a solution to set into a jelly on cooling. Spread 
over a surface gelatine dries with a fair amount of lustre. The 
dry surface is, however, sensitive to the action of water; but by 
mixing a small proportion of potassium chromate or bichromate, 
and afterwards exposing the surface to the action of light, the 
gelatine is rendered insoluble in water; by taking advantage of 
this property waterproof varnishes may be made from gelatine. 
It only requires about | to 1 per cent, (of the weight of the 
gelatine) of the chromates to effect this change. Gelatine is 
insoluble in alcohol, and the addition of that body to an aqueous 
solution causes the precipitation of the gelatine in the form of a 
viscid adhesive mass. Tannin likewise precipitates gelatine from 
aqueous solutions as also do solutions of various gums. Gelatine 
solutions, if kept, decompose and evolve a putrid odour; this 
decomposition may be prevented by the addition of a few drops 
of some antiseptic, such as thymol, carbolic acid, salicylic acid, &c. 

Albumen. Albumen is obtained from the white of eggs or 
from blood-serum by a process of drying, when it forms horny 
masses of a pale yellow colour and high lustre. Egg albumen is 
the best of the two varieties, having a whitish colour and the 
highest lustre. Albumen is soluble in cold water; but ordinary 
commercial samples generally contain a small quantity of in- 
soluble albumen, due to their being dried at rather too high a 
temperature. The characteristic property of albumen is that 
when the aqueous solutions are heated the albumen is coagulated 
and falls as a white insoluble mass; this property is exceedingly 
characteristic of albumen, and upon it much of its application in 
calico-printing, &c., is based. It is also precipitated from its 
solutions by various metallic salts and by alcohol. Its aqueous 
solutions are very liable to decompose, giving off small quantities 
of sulphuretted hydrogen, &c. ; this decomposition may be pre- 
vented by the addition of antiseptics, as in the case of gelatine. 
Albumen occurs in some kinds of water varnishes which are almost 
entirely composed of it as it does not work well with other bodies. 

4th. SOLVENTS. These have already been sufficiently 
described (see pp. 360 to 383). 



420 VARNISHES. 

5th. DRIERS. These also have been previously dealt with 
(see p. 386, et seq.). They are used in making oil varnishes 
for the same purpose as in paint making i.e., to make the 
varnish dry quicker. They should be used very sparingly, as 
they do not add to the durability of the varnish at all, their 
tendency being to make the coat harder, but more liable to crack, 
while they reduce the lustre and transparency. 

6th. COLOURING MATTERS. The materials which may 
be used in the manufacture of varnishes for the purpose of 
colouring them are very numerous, and the number has been 
much increased of late years by the introduction of the coal-tar 
colours. They may be divided into two groups : 1, Natural 
colouring matters, and 2, artificial colouring matters ; the latter 
group is now by far the most numerous. 

1. NATURAL COLOURING MATTERS. There are a 
fairly large number of these bodies at the disposal of the varnish 
maker ; but he cannot use all the colouring matters which may, 
or can, be obtained from natural sources, as one important pro- 
perty which varnish colours must possess, viz., solubility in oil 
and spirit, is not possessed by many, as, for example, indigo, 
hsematoxylin, and the colouring matter of logwood. The follow- 
ing products can be used for colouring varnishes : 

Turmeric. This natural yellow dyestuff is the rhizome or 
root-stem of several species of Curcuma; C. longa, C. tinctoria, and 
C. rotunda being those which yield the largest portion of the 
turmeric of commerce. They are found in many parts of Asia, 
but the principal supply comes from Bengal and Madras in India; 
some comes from China, Siam, and Singapore. Turmeric is usually 
sold in the form of root-shaped masses, some varieties being more 
or less branched ; these are externally of a greyish and internally 
of a deep orange-yellow colour ; in taste they are somewhat bitter 
and hot. The colouring matter is but slightly soluble in cold, but 
more freely soluble in hot water Alcohol dissolves the colouring 
matter from turmeric as well as some resinous matter. An analysis 
of turmeric root shows it to contain 11 to 12 per cent, of colouring 
matter, 10 to 11 per cent, of a yellow-brown resin, 1 per cent, of a 
volatile oil, 14 per cent, of gummy matter, 57 per cent, of starch, 
soluble salts, &c., and about 7 per cent, of water. 

The colouring principle of turmeric is called curcumin, a body 
having the formula C 14 H 14 O 4 . It can be obtained from turmeric 
by digesting the powdered root with water until n6 more colour 
is extracted, evaporating down the solution to dryness, extracting 
with benzene and purifying by crystallisation from alcohol. Other 
methods for extracting it can be adopted. Curcumin crystallises 



NATURAL COLOURING MATTERS. 421 

from hot alcohol in needle-shaped prisms, having an orange-red 
colour with a blue reflection, is odourless, and melts at 178 C. 
It is slightly soluble in water, but more readily soluble in alcolTol, 
especially when boiling ; it is also soluble in wood spirit and in 

tlacial acetic acid ; it is practically insoluble in petroleum ether, 
trong sulphuric acid dissolves it ; at first the solution has a 
reddish purple colour, but this soon changes to black owing to 
charring taking place. In solutions of alkalies, either the caustic 
or carbonate, curcumin dissolves with a reddish-brown colour. 
Curcumin is very sensitive to alkalies, and hence is often used as 
a test for alkalinity, the merest trace of alkali being sufficient to 
turn a solution of turmeric from yellow to brown. 

A very characteristic reaction for turmeric and its colouring 
principle, curcumin, is the red colour which it gives with boric 
acid, which colour is quite distinct from that given by alkalies. 
This colour is produced when an aqueous solution of boric acid is 
added to an aqueous solution of turmeric. A solution of borax, 
to which hydrochloric acid has been added in sufficient amount 
.as to make the solution acid to litmus, may be used instead of 
the boric acid. Paper coloured with turmeric tincture, dried, 
and then moistened with the acidified solution of borax, shows 
this reaction very well, but still better if it be again dried. 

Turmeric root is rarely adulterated, but the turmeric powder 
is sometimes mixed with starch and mineral matters. The latter 
can be detected by the increase in the amount of ash left on 
burning ; this ash should not be more than 5 per cent., and an 
analysis of it will soon show whether any mineral matter has 
been added. Starch is best detected by the microscope and by 
the fact that its addition materially reduces the proportion of ash 
and increases the proportion of soluble matter. 

Turmeric is used to give a yellow colour to varnishes and 
stains, which stain is tolerably permanent. 

Gamboge. Gamboge is a gum-resin, the product of Garcinia 
morella, a tree growing in Siam, from whence gamboge is ex- 
ported in large quantities. It usually occurs in cylindrical rolls 
of about an inch or an inch and a quarter in diameter, and 
of varying length ; these rolls, which are of an orange colour 
externally, are made by melting the gum-resin, and pouring 
the melted material into moulds made of bamboo cane. It is 
rather brittle, and the pieces of gamboge are often covered with 
a yellow powder. When broken it exhibits a conchoidal fracture 
with a vitreous lustre. When viewed through the edges it is 
more or less transparent. Its solution in water is rather cloudy, 
and of a yellow colourj; its taste is but slight at first, but an 



422 VARNISHES. 

acrid after-taste is perceived. It possesses strong purgative pro- 
perties, and, hence, is frequently used in medicine. 

An analysis made by the author shows it to contain 

Moisture, 2 '50 per cent. 

Mineral matter, ..... 1*05 ,, 

Resin, soluble in ether, . . 66 '05 ,, 
Wax, soluble in alcohol, . . . 4'31 

Gum, 26-03 

99-94 

Gambogin, or gambogic acid, the resin of gamboge has, ac- 
cording to Buchner, the formula C 30 H 35 O 6 . It is obtained in a 
rather impure condition by treating gamboge with ether; on 
evaporating off the ether the gambogin is left behind as a trans- 
parent, vitreous, and brittle mass, breaking with a conchoidal 
fracture. In colour it is a reddish-orange, forming a yellow 
powder. When heated it softens, and melts at about 75 to 80 
C. ; on cooling, the mass again becomes solid. It has no odour 
nor taste, and is destitute of purgative properties. Gamboge 
resin is readily soluble in ether, alcohol, and chloroform ; petro- 
leum spirit has but a slight solvent action on it. It readily 
dissolves in alkaline solutions. The carbonates of the alkalies 
are decomposed by it on boiling, with evolution of carbonic acid, 
showing that it has acid properties. The solutions have an 
orange-red colour; when acidulated they deposit the resin in 
flakes. 

The wax, which is insoluble in ether, but soluble in alcohol, 
is a soft brownish substance, melting readily at a low tempera- 
ture ; it has a bitter taste, which is very persistent, and a slight 
purgative action. Caustic soda dissolves it with a brownish- 
yellow colour ; the addition of acids reprecipitates it. 

The gum is a transparent brownish mass, having a sweet 
taste, and but slightly adhesive properties. It dissolves in 
water, forming a somewhat opalescent solution, which, on adding 
an acid, becomes clear. It is not precipitated by basic lead acetate, 
mercuric chloride, ferric chloride, borax, or alcohol. It seems to 
be a glucoside, yielding a sugar on hydrolysis. 

Gamboge is soluble in alcohol, partially in ether, readily in 
ammonia. It is used as a pigment in water-colour painting, and 
also for colouring varnishes. 

Dragon's Blood. This is a product of Calamus draco, the 
tree which yields the rattan canes of commerce, and a native of 
Eastern Asia, from which country this resin is sent in large 
quantities to Europe. The fruit of this tree, on approaching 



NATUEAL COLOURING MATTERS. 423 

maturity, becomes covered with a red resinous mass of a friable 
character; this resin is collected by shaking the fruits into 
baskets, and then sifting the resin from the stems and particles 
of woody fibre. The resin is then melted either by the heat of 
the sun, or by that of boiling water, and cast into rolls by 
wrapping it in a palm leaf; in this form it is sent into commerce, 
being exported into this country from Singapore, Batavia, and 
other Asiatic ports. Dragon's blood occurs in the form of 
roughly cylindrical sticks about 13 to 14 inches long, and about 
J to 1 inch thick ; but smaller pieces are by no means un- 
common. Externally, the resin has a blackish- brown colour ; 
but viewed in thin slices it is transparent, and of a deep crimson 
colour. Its specific gravity is about 1'2, varying a little, the 
best qualities always having the highest specific gravity. Its 
taste is sweet when pure, but adulterated samples often have an 
acrid taste. 

It is soluble in alcohol, benzol, chloroform, carbon bisulphide, 
petroleum spirit, shale spirit, glacial acetic acid, caustic soda, and 
some other solvents ; it is only slightly soluble in ether, and is 
all but insoluble in turpentine. It melts at about 120 C. 
(248 F.), and when heated evolves benzoic acid. It is used for 
colouring spirit varnishes a deep red, and for this purpose is one 
of the best colouring matters which can be used. 

Besides the above, a few other varieties of dragon's blood are 
met with in commerce to a small extent. Socotran dragon's bloody 
the resin of the Draccena Ombet or of D. schizantha, from which 
trees the natives obtain the resin by making incisions in the 
bark and collecting the resin which exudes. This variety comes 
into the London market from Bombay and Zanzibar in the form 
of drops or tears. An exactly similar product is obtained in the 
Somali country. 

Canary Islands dragon's blood is the variety obtained from 
the celebrated dragon tree of TenerifFe and adjacent islands, 
Draccena draco. Very little of this now finds its way into 
European commerce, but it is used locally for various purposes. 
Mexican dragon's blood is obtained from Croton draco, but is 
rarely met with in English commerce. 

Gum Accroides, also known as black-boy gum, Xanthorrhsea 
resin, &c. The resins yielded by the several species of Xanthorrhcea, 
natives of Australia, have of late years been imported to a large 
amount for varnish making. Some six or seven species of Xan- 
thorrhcea are known to yield resin in large amount; in the season 
the resin exudes in no inconsiderable amount, but by crushing 
the stems and sifting further quantities are obtained. The prin- 



424 VARNISHES. 

cipal species are Xanthorrhcea australis, which grows in parts of 
Victoria, X. hastilis and X. arborea. The resin is distinguished 
as red or yellow; by some authorities these have been ascribed 
to different trees; on the other hand, it is possible that the red 
may simply be derived from the yellow by exposure to the air. 
All the varieties of gum accroides are soluble in alcohol with a 
strong yellow colour. The odour is pleasant, and resembles that 
of benzoin. Treated with nitric acid it yields a large proportion 
of picric acid. Its specific gravity is 1*197. 

2. COAL-TAB, COLOURS. This is a very large and varied 
group of colouring matters, many of which have come largely into 
use during late years for colouring varnishes and making the so- 
called enamel paints. It is not necessary to deal with the chemical 
nature of these colouring matters here, the reader is, therefore, 
referred to the author's Dictionary of Coal-tar Colours, or to 
Benedikt & Knecht's Chemistry of Coal-tar Colours, for details 
on this subject. 

The coal-tar colours can be divided into four groups : 

1. The Benzidine colours, whose characteristic feature is that 
they will dye unmordanted cotton from a boiling bath of salt or 
other alkaline compound. As a rule, this class of colours, which 
includes Congo red, benzopurpurine, chrysamine, benzoazurine, 
Titan pink, Titan yellow, diamine-yellows, &c., are of no use as 
colouring agents in varnish making, as their solubility in alcohol, 
oil, and other varnish solvents is but slight ; not only so, but many 
of them, the reds especially, are rather fugitive to light, and are 
readily acted upon by any substance having weak acid properties. 

2. Basic Colouring Matters. This is a numerous group of 
colouring matters, many of which are of great use in varnish 
making. The group includes magenta, safranine, Bismarck-brown, 
chrysoidine, phosphine, rhodamine, chinoline- yellow, auramine, 
induline, nigrosine, soluble blue, methyl-violet, Hofmann violet, 
benzyl violet, brilliant green, methyl-green, &c. All these colours 
are soluble in water and in alcohol. They can, therefore, be used 
for colouring water- or spirit- varnishes. Magenta gives crimsons, 
but should be used in comparatively dilute solutions, say of a 
strength of about 1 per cent., as when stronger the solutions are 
apt on drying to leave a bronzy colour behind, and not a red. 
Safranine can be used for scarlets, especially if mixed with a little 
auramine. Phosphine can be used for oranges, but it is rather 
expensive. Rhodamine can be used for pink varnishes. Aura- 
mine dyes yellows. Induline, nigrosine, and another product 
generally known as brilliant black, belong to a peculiar group of 
dyestufi's which exist in two forms; one, soluble in water and 



ARTIFICIAL COLOURING MATTERS. 425 

almost insoluble in spirit; the other, insoluble in water, but 
soluble in spirit; this latter variety is distinguished as induline 
spirit soluble, or nigrosine spirit soluble, as the case may be. 
Brilliant black is only made in the spirit soluble form. These 
dyes are very useful to the varnish maker. Induline gives dark 
blues ; there are several brands of induline giving various shades 
of blues. Nigrosine can be used for greys, and when used in 
large proportion gives very good blacks. Brilliant black gives 
blacks; these, like induline, are made in various brands, giving 
different shades of greys and blacks, some bluish, others greyish 
in tone; a varnish maker should obtain samples from various 
makers, and use that which gives him the best results as to 
strength of colour and tone. The other colouring matters named 
above are sufficiently indicated by their name; the violets are 
made in several shades, from a red to a blue-violet ; like magenta, 
they should not be used in too strong a solution or they dry up 
with a bronzy cast ; the greens are of a blue shade, but by mixing 
them with a yellow, a pure green or a variety of different shades, 
from a blue- to a yellow-green, can be readily obtained. 

These colouring matters do not readily dissolve in oil or oil 
varnishes, to which they give, as a rule, but a faint coloration, 
but the bases of these basic dyestuffs are soluble in oil without 
much difficulty; these bases are made on a limited scale by 
Messrs. Read Holliday & Sons, and other makers, and sold under 
the name of oil-yellow, oil-scarlet, oil-green, &c., expressly for 
the purpose of colouring oils and fatty matters in general. 

3. Acid Colouring Matters. This group of dyestuffs is charac- 
terised by its dyeing wool and silk direct in an acid bath ; hence 
the name "acid" colours. It is divisible into several sub-groups 
depending upon the chemical composition and relations of the 
dyes, viz. : 

a. The JZosins, a class of colouring matter derived from fluo- 
rescein and characterised by giving scarlets of various hues of a 
very bright character; they are, however, rather fugitive. They 
are readily soluble in water and alcohol, and their solutions have 
a fluorescence of a more or less pronounced character. Eosin G 
has a yellowish-green, and erythrosine a trifling fluorescence; 
safrosin, faint yellowish-green; phloxine, light green; rose Bengal 
has no fluorescence in an aqueous solution, but an alcoholic solu- 
tion has a golden yellow fluorescence; erythrin, green-yellow; 
eosin J, strong green-yellow; cyanosin, orange-yellow; chrysolin, 
yellowish- green; the fluorescence is in each case strongest in the 
alcoholic solution. Although the eosins can be used for colouring 
spirit varnishes, yet they are not satisfactory in use, as they have 
a tendency^to dry with a yellowish bloom or fluorescence, which 



426 VARNISHES. 

obscures their real colour. The eosins are sold under a variety 
of other names. 

b. Nitro-colouring matters, such as naphthol-yellow, aurantia, 
picric acid, citronine, &c., which are more or less of a yellow or 
orange colour, and are generally soluble in water and spirit ; they 
are fairly permanent and can be used with advantage in the 
colouring of varnishes. 

c. Acid colouring matters, such as acid magenta, acid green, 
acid yellow, acid violet, &c., which are mostly derivatives of the 
basic colours made by the process of sulphonation ; they are 
readily soluble in water, but not all are soluble in spirit. They 
can be used for colouring varnishes. 

d. Azo- colours. This is a very numerous group of coal-tar 
dyestuffs mostly of a red, orange, or yellow colour, very few azo- 
blues, greens, or violets being known. The scarlets, croceine 
scarlets, ponceaus, azo-yellow, azo-violets, oranges, naphthol blacks 
and green, and most of the new blacks belong to this group of 
dyestuffs. The azo-dyes are, however, too numerous to give a 
detailed list of their names. These colouring matters are nearly 
all soluble in water \ some are soluble in spirit, as shown in the 
list given below, and may be used for colouring varnishes, since, 
as a rule, they give very bright colours possessing a fair degree 
of fastness on exposure to light and air. 

4. Adjective Dyestuffs. This class of colouring matters is char- 
acterised by the members requiring the aid of a second substance, 
the mordant, to fix them on the fibre and to develop the colour 
from the dyestuff. It includes alizarine and all the so-called 
alizarine dyes, gambine, coerulein, gallein, gallocyanine, gallo- 
flavine, &c. As colouring matters for varnishes they are quite 
useless, as they do not possess, as a rule, the essential feature of 
being soluble in water or spirit. 

The following tables of the solubility of the coal-tar colours in 
various media will be found very useful by the varnish maker as 
showing him what can and what cannot be used for colouring 
varnishes : 

1. Colouring matters soluble in water and alcohol. 



Magenta. 


Acid magenta. 


Vesuvine. 


Iodine green. 


Cardinal. 


Cerise. 


Rubin. 


Crimson. 


Erythrosin. 


Rose Bengal. 


Methyl eosin. 


Eosin. 


Safranine. 


Coccine. 


Phloxine, 


Corallin. 


Methylene blue. 


Peacock blue. 


Navy blue. 


Benzyl blue. 


Picric acid. 
Curcumine. 


Naphthol yellow. 
Brilliant yellow. 


Phosphine. 
Auramine. 


Aurantia. 
Chrysophenine. 


Metanil yellow. 


Methyl orange. 


Chrysoidine. 


Mandarin. 


Hofmann violet. 


Kegina purple. 


Methyl violet. 


Acid violet. 


Malachite green. 


Brilliant green. 


Acid mauve. 


Methyl green. 


Bismarck brown. 









VARNISH MAKING. 



427 



2. Colouring matters soluble in water 


only. 


Congo. 


Heliotrope. 


China blue. 


Brilliant scarlet. 


Congo corinth. 


Brilliant blue. 


Regina violet. 


Acid yellow. 


Brilliant Congo. 


Wool blue. 


Azo- violet. 


Resorciu yellow. 


Benzopurpurine. 


Black blue. 


Fast brown. 


Quinoliue yellow. 


Delta purpurine. 


Benzoazurine. 


Acid brown. 


Azo-acid yellow. 


Roseazurine. 


Azo-blue. 


Resorcin brown. 


Naphthol yellow. 


Hessian purple. 
Fast red. 


Guernsey blue. 
Hessian blue. 


Guinea green. 
Aniline grey. 


Chrysamine. 
Hessian yellow. 


Archil red. 


Water blue. 


Nigrosine. 


Curcumein. 


Ponceau. 


Bavarian blue. 


Silver grey. 


Orange. 


Scarlet. 


Capri blue. 


Wool black. 


Methyl orange. 


Azo-rubine. 


Alkali blue. 


Nacarat. 


Rubin S. 



3. Colouring matters soluble in alcohol only. 

Rosaniline base. Aurine. Soudan. Spirit blue. 

Nigrosine spirit Malachite green Brilliant black. Induline spirit 

soluble. base. soluble. 

Humboldt blue. New violet. Auramine base. 



4. Colours soluble in oil. 



Rosaniline base. 
Magenta base. 
Oil yellow. 
Butter yellow. 



Violet base. 
Auramine base. 
Oil violet. 
Oil brown. 



Soudan I. 
Picric acid. 
Oil orange. 
Oil scarlet. 



Soudan II. 
Oil green. 
Oil crimson. 



Practically none of the coal-tar colours are soluble in petroleum 
spirit, turpentine, or benzol. While, therefore, the coal-tar colours 
are available for colouring water- and spirit- varnishes, but few of 
them are useful for colouring oil- varnishes, and none for colour- 
ing varnishes made from turpentine, petroleum spirit, or benzol. 



VARNISH MAKING. 

For practical purposes the following simple classification into 
four groups is quite sufficient : 

1st. Natural varnishes. 
2nd. Oil varnishes. 
3rd. Spirit varnishes. 
4th. Water varnishes. 

The first group comprises the natural lacquers of India, China 
and Japan, which issue from the tree in a liquid form, and only 
require applying to the prepared object to varnish them. The 
second group consists of the artificial varnishes made from resins 
of various kinds dissolved in oil ; these form very useful varnishes 



428 VARNISHES. 

and are applied to all kinds of objects. The third group includes 
the varnishes made by dissolving resins of volatile spirits of 
various kinds; on application to the object the spirit evaporates 
and leaves the resin behind as a lustrous coat. The fourth group 
embraces a few varnishes made by dissolving resins and gums in 
water; they are not numerous and are only used for special 
purposes. 



1st. NATURAL VARNISHES. 

In India, China, and Japan are found a number of trees of 
various species whose sap or juice has the property of drying 
hard with some considerable lustre; the sap is collected and used 
for a varnish in the countries named ; none of it is exported to 
this country. The natural varnish of Japan is the variety best 
known in this country; it is used for producing the famous 
lacquer ware of the Eastern Empire. 

1. Japan Lacquer. The best account of this material has 
been published in the form of a Consulate Report by Mr. 
John J. Quinn, the acting Consul at Hakodate, ^1882, on the 
" Lacquer Industry of Japan," from which the following brief 
description has been taken: 

The Japanese lacquer is the product of the Rhus vernicifera, a 
tree found growing all over the main island and, in smaller 
quantity, in the other islands of the Japanese group. The 
lacquer trees are cultivated, and some companies have been 
formed to plant waste lands with the trees ; they may be grown 
from seed sown about February, the young trees being trans- 
planted when about a year old. 

It takes ten years to raise a tree from ; seed to the lacquer 
yielding state, and even then it only gives about 3 ozs.; by grafting, 
the trees can be made to grow quicker, and then a ten-year old 
tree will yield about 4| ozs. of lacquer. There are some differ- 
ences in the quality of the lacquer yielded by trees grown in 
different localities ; that coming from Tsugaru, Nambu, Akita, 
and Aidzu is the best and most transparent, and is the variety 
most in use in Kioto and Osaka, which are the great centres of 
the lacquer-ware industry. 

In June of each year tappers go round the plantations, and, 
after certain preliminary operations, such as clearing the ground 
and marking the trees, makes a number of cuts on the trees; 
from each of these cuts sap exudes and is collected. The quality 
of the sap obtained from the cuts varies somewhat ; that from the 



NATURAL VARNISHES. 429 

first five cuts is poor, containing a good deal of water ; that from 
the middle fifteen cuts is the best; that from the last five cuts is 
thin. As a rule the tapping of the tree kills it, although, by not 
giving quite so many cuts, a second year's flow of sap can be 
obtained, but such sap is not so good as the first year's sap, so 
that a tree is rarely tapped twice. The custom is for the lacquer 
merchants to buy the right of tapping the trees from the farmers ; 
the latter still keeping possession of the tree, and when it has 
been tapped generally cut it down for firewood or for making 
cabinet ware, &c. 

The lacquer obtained from the trees is known as Ki-urushi or 
tree lacquer, and Seshime or branch lacquer, these terms indicat- 
ing from which part of the tree the lacquer comes. The lacquer 
merchants buy this lacquer from the tappers, and then treat it in 
various ways for sale to the lacquer-ware makers. The crude 
lacquer contains some water, and in preparing it for use this 
water has to be eliminated, which is done by allowing the lacquer 
to stand in vessels exposed to the rays of the sun for some time; 
the operation is facilitated by adding a small quantity of water 
to the crude lacquer. 

Crude lacquer is a liquid of about the colour and the consistence 
of cream ; if exposed to the sun for some days it loses its colour and 
becomes quite black or nearly so, but at the same time becomes 
more or less transparent, and will not dry when used for lac- 
quering articles; if, however, some water be mixed with it two 
or three times a day for two or three days, and this extra water 
be evaporated away in the usual manner, then the lacquer 
acquires the property of drying and may be used for lacquering. 
Branch lacquer does not dry so quickly as tree lacquer, and is, 
therefore, rarely used alone; thence mixtures of the lacquer are 
made with other ingredients and sold under a variety of names 
for special kinds of lacquer ware. 

Old lacquer is more difficult to treat than new lacquer. One 
of the most peculiar properties of Japan lacquer is that it will 
not dry in a dry atmosphere, but it requires a damp one to become 
hard. 

As might be expected the lacquer varies in composition; this 
may arise partly from natural causes and partly from the fact 
that the lacquer merchants rarely sell the lacquer in the same 
condition as they obtain it, but generally mix it with other 
bodies. Two analyses of Japanese lacquers have been published. 
Mr. S. Isima Ishimatsu gives the following as the composition of 
a sample of Yoshino lacquer, which is one of the best varieties : 

The specific gravity at 20 C. was 1 '002 and it contained 



430 VARNISHES. 

Urushic acid, . . . . . 85'15 per cent. 
Gum arable, ..... 3'15 ,, 

Nitrogenous matter, . . . . 2 '28 ,, 

Water and volatile matter, . . 9 '42 ,, 

Another sample analysed by Mr. J. Takayma had the com- 
position 

Urushic acid, 64'07 per cent. 

Gum arable, . . . . . 6 '05 ,, 

Nitrogenous matter, . . . . 3 '43 

Oil, 0-23 

Water and volatile matter, . . . 26 '22 ,, 

The higher the proportion of urushic acid contained in the 
lacquer, the better is the quality of the article. Urushic acid 
contains 77'05 per cent, of carbon, 9O1 hydrogen, and 13-94 
oxygen, corresponding to the formula C 22 H 31 O 3 . 

Chinese lacquer is identical with Japanese lacquer. China, 
indeed, is the home of the lacquer industry, from whence it spread 
to Japan, where it has become of more importance than in its 
original home. 

Indian and Ceylon Lacquer. From several trees, natives of 
India and Ceylon, is obtained the natural varnish used by Indian 
and Cingalese artificers in the manufacture of lacquer ware. In 
Ceylon the lacquer comes from a species of Semecarpus, which 
grows tolerably luxuriantly all over the island. In India several 
trees yield the lacquer, viz., Semecarpus anacardium, which grows 
in the Concan, Coromandel, Courtallum, Gujerat, Bengal, and 
Travancore districts j and Semecarpus travancorica, which grows 
in the damp forests of the Travancore and Tinnevelly mountains. 
In the Western Ghauts of Madras and Bombay and in Bengal 
lacquer is obtained from Holigarna longifolia, which exudes the 
juice from fissures in the bark \ while in Semecarpus it comes 
from the pericarp. After exuding from the tree it is collected 
and sold to the lacquer- ware makers ; it readily hardens on 
exposure and assumes a black colour in so doing ; as a varnish it 
is very good, being lustrous, transparent, hard, and strongly 
adhesive. 

Burmese Lacquer. This lacquer, which is used in large 
quantities in the preparation of Burmese lacquer ware of all kinds, 
is obtained from every part of the kheu tree, MelanorrJicea 
usitatissima, which grows very abundantly in every part of the 
country, but more particularly in the Kubbu valley where it 
attains its greatest size j on the sea coast it is usually of somewhat 
dwarfish growth. The varnish is collected during January to 



MANUFACTURE OF OIL VARNISHES. 431 

April. The collector makes wounds in the bark of the tree and 
thrusts into them short lengths of bamboo, in which the juice 
collects ; as the bamboos get filled they are emptied and replaced ; 
there will often be as many as 100 bamboo canes in a single tree, 
the usual yield of which is about 3^ Ibs. of lacquer. Burmese 
lacquer is a thick, very viscid liquid having a turpentinous odour ; 
in colour it is grey, but on exposure it soon becomes black. It 
dissolves freely in alcohol, benzene, and turpentine. It dries hard 
and lustrous, but rather slowly. Its drying is facilitated by 
mixing a little gold size or other varnish with it. Gingelly oil is 
often added to the lacquer. 



2nd. MANUFACTURE OF OIL VARNISHES. 

Oil varnishes are divided commerciall